UPDATED DEC. 18: "Dark Horse" has been released by Capitol Records as the official third single from Perry's album "PRISM" at pop and rhythmic radio. Below is the backstory of how the song drew unsolicited early radio support, helping spur its release as a single.

Katy Perry's song "Dark Horse" is living up to its title as a surprise hit at top 40 radio.

Having been released less than two months ago, Perry's No. 1 Billboard 200 album "PRISM" this week yields its third hit on the Nielsen BDS-based Mainstream Top 40 radio airplay chart, as "Dark Horse," featuring Juicy J, debuts at No. 40.

Unlike uptempo first single "Roar," which topped the tally for five weeks, and ballad follow-up "Unconditionally," which lifts 10-8 this week (although down seven plays from last week), the moodier, rap-infused "Horse" trots in due solely to unsolicited airplay.

Thirty-nine of the 164 Mainstream Top 40 reporters played "Dark Horse" in the Dec. 2-8 tracking week (880 plays, up 38%), even though Capitol Records isn't actively seeking spins for it.
 

 
Notably, "Dark Horse" was released as a digital "PRISM" preview track and debuted on the Hot Digital Songs chart at No. 4 the week of Oct. 5. It's sold 785,000 downloads to date, according to Nielsen SoundScan. ("Unconditionally," meanwhile, has sold less than half that sum: 340,000. The set's other preview cut, "Walking on Air," which bowed on Hot Digital Songs at No. 8 with 113,000 the week of Oct. 19, has sold 150,000 so far.)

Such exposure, and a chart position, for a song not being promoted to top 40, while another track from an artist is a focus track, is a rarity at the format. The practice has a deeper history, for instance, at country radio, with superstars like Shania Twain having charted cuts on Country Airplay in the '90s prior to their releases as official singles. Twain's "From This Moment On," for example, dotted the chart between Nos. 75 and 62 for 17 weeks before its proper release and eventual vault to a No. 6 peak.

Meanwhile, after logging five Country Airplay top 10s from his 1993 album "In Pieces," Garth Brooks logged a 20-week chart run in 1994-95 with the No. 49-peaking "The Red Strokes." More recently, Carrie Underwood followed up three No. 1s and a No. 2 hit from her album "Play On" with a No. 38 peak for "Songs Like This" in 2011. Neither ever a promoted single, "Red" and "Songs" charted entirely due to play by adventurous programmers.

Also in 2011, Bruno Mars' "Marry You" received notable airplay at mainstream and adult top 40 radio, even though it, too, was never promoted as a designated single. It's sold 1.9 million downloads, an astounding sum for an album cut. (A cover by the cast of Fox's "Glee," which has sold 384,000, helped pad the song's profile.)
 
Clearly, it's no coincidence that star acts have a chance to occasionally impact airplay charts with deeper cuts off hit albums. It's perhaps tougher at top 40 than country, since it's harder to gain consensus at the former format, which culls hits from a variety of genres (pop, R&B, rock, dance, country, etc.) I.e., it's hard enough for a label to score a Mainstream Top 40 hit with a song it's promoting. It's all the more impressive when a song charts there that a label isn't even working.
 
RADIO: 'DARK HORSE' A 'WELCOME RHYTHMIC SOUND'
 
Stations playing "Dark Horse," unsurprisingly, say that Perry's star power is fueling its exposure.
 
It's not, however, the only reason.
 
Danny Howard, program director of WDOD Chattanooga, Tenn., which played "Unconditionally" 70 times and "Dark Horse" 17 times last week, says that by spinning "Unconditionally" and "Dark Horse," listeners get to hear different sides of Perry's musical personality. "Katy is possibly one of only a handful of artists, perhaps in history, that could pull this off. We look at 'Roar' and 'Unconditionally' as part of her signature pop sound, while 'Dark Horse' is her more edgy, wild alter ego. It's a welcome rhythmic sound in the sea of pop and alternative material out now."
 
Howard adds that strong sales for "Dark Horse" have increased his confidence in playing it. "While Capitol hasn't actively worked the track, it's a top 20-selling song in Chattanooga, which I'm sure [the label isn't] too upset about. Plus, 'Unconditionally' is in super-power rotation. So, everyone is smiling."
 
Nick Russo, assistant PD/music director of KTFM San Antonio agrees that says that the sound of "Dark Horse" has helped make it a surprise addition; the station played the track 41 times last week, compared to giving "Unconditionally" 10 plays. "[KTFM is] a rhythmic-leaning pop station and we became convinced that 'Dark Horse' was a hit record. Juicy J is very familiar to our audience and the sound is something we look for with KTFM.
 
"Audience research has come back huge. While I hope it grows nationally, whether or not it does, it's a win for us."
 
WJFX Fort Wayne, Ind., likewise played "Dark Horse" (62 times) more than "Unconditionally" last week (37). PD Brooke Taylor concurs that the station is following its audience's reaction to the former song. "While we understand that it's not the label priority, the fans of our brand are the ones we have to take into consideration when it comes to our title selection."
 
Sound, too, Taylor says, is central to the station's choice to wrangle "Dark Horse" onto its playlist. "Sonically, it has the architecture of what, on the surface, sounds like a hit from a proven superstar. After exposing it to the market, our belief rang true. From Shazam rankings to retail to research, 'Dark Horse' has actually outperformed 'Unconditionally.'"

"It also might be forgotten that 'Dark Horse' was a fan-selected favorite in a recent Pepsi/MTV Video Music Awards promotion," Taylor points out.
 
"If her fanbase already picked it as one of its favorites, why would radio, and Capitol, not follow suit?"
 
CAPITOL: 'AN EMBARRASSMENT OF RICHES'
 
Ultimately, Capitol Music Group executive VP Greg Thompson feels that the benefits of unforeseen airplay for "Dark Horse" outweigh the possibility that the song is infringing on the popularity of "Unconditionally," and, thus, potentially throwing a wrench into the label's long-term plan for working singles from "PRISM." (The set has sold 660,000 copies in the U.S. in its first six weeks. Last week, owed to busy Thanksgiving week/Black Friday consumer activity; sale-pricing for the album in that period; and Perry's performance of "Unconditionally" on the American Music Awards [broadcast on ABC on Nov. 24], "PRISM" roared [pun intended] by 252% to 136,000 sold in the SoundScan tracking week, good for Greatest Gainer honors on the chart.)
 

 
Thompson calls the confluence of the songs' airplay an "embarrassment of riches."
 
"Katy's become one of the most consistent hit-makers for top 40, so the reality is once radio heard 'PRISM,' it reacted," he says. "The album is packed with singles and, currently, three of its songs are in the iTunes Store's top 20.
 
"'Roar' is one of the biggest songs of the year, with long shadow, still selling well and going strong at radio; 'Unconditionally' is top 10 at top 40; and, people love 'Dark Horse,' in part, I think, because it shows a different side of 'PRISM.' For some, that's an exciting slice.
 
"You can't stop people's enthusiasm just because a song is a non-focus track."
 
Thompson realizes that the laws of music-scheduling mean that if a station is playing "Dark Horse," even if it's also playing "Unconditionally," the latter could lose potential spins, since artist separation dictates that there's only so much of any artist to go around. "'Dark Horse' could cancel out maximizing the chart position of 'Unconditionally,' " he concedes.
 
Still, Capitol, Thompson and Perry herself hope that "Unconditionally" "gets its full day in court," as Thompson says. He cites that its momentum has boosted with the release of its official video, which, since its Nov. 20 premiere, has drawn 18 million worldwide YouTube views. "The video is off to a great start, and it follows her performance of the song on the American Music Awards.
 
"And, on a personal, lyrical level, 'Unconditionally' is very special to Katy."
 
The song additionally bullets at No. 10 on Adult Top 40 (up 4% in plays) and re-enters Adult Contemporary at No. 30 (up 33%).
 
Now that "Dark Horse" has galloped onto the Mainstream Top 40 chart, a natural question follows its unexpected ride to radio, as well as its lofty sales: Will Capitol choose it as the third official single from "PRISM"?
 
"Katy's well aware of 'Dark Horse' and we're discussing it," Thompson says. "It might be the next single. Or, due to the play it's getting now, maybe it can never have a full campaign as a single. We're watching it – and certainly appreciating the airplay – while still wanting 'Unconditionally' to have a full run."
 
"'Dark Horse' might cost 'Unconditionally' some plays, but, outside of that, there's a lot of Katy Perry playing on radio stations in America," Thompson says.
 
"That's a good thing."