On Gossip Girl, Blake and Penn played on-and-off couple Serena and Dan, and the actors also dated on-and-off IRL between 2007 and 2010. Both Blake and Penn remained on the series until its finale in 2012, and their characters even ended up getting married.
During a May 2013 interview with Elle, Penn was asked whether it was difficult to work with Blake after their breakup, and Penn said they were "ultimately professional" together, adding, "We were very much caught up in the show, which itself was a six-year endurance test. Our relationship was a part of that and helped us through it. I mean, like anything valuable, it was good and it was bad and it was a learning experience."
Later, when Penn appeared on Watch What Happens Live with Andy Cohen in March 2015, a caller asked the actor which Gossip Girl cast member enjoyed kissing the most onscreen and who he enjoyed kissing the least. His answer for both: Blake. "I'd say best . . . I'll say it was Blake, because we actually had a relationship at the time," Penn said. He then jokingly adding that the worst kiss was "maybe Blake after we broke up," though he noted that they both handled the situation well. "We should both pat ourselves on the back because anything is complicated in that way . . . and we handled it," he added.