Why Blue Bloods Spin-Off Boston Blue Recast Sean Reagan

"Blue Bloods" spin-off "Boston Blue" sees Donnie Wahlberg's Danny Reagan take the lead in this continuation of the Reagan family's story. But the show is debuting amid some less-than-ideal circumstances, with former "Blue Bloods" star Tom Selleck talking openly about his upset with the mothership series coming to a close in what is easily one of the most controversial TV show cancellations of recent years. Now, Sean Reagan — Danny's younger son — has been recast for the spin-off. Why? Well, it's not entirely clear, but according to the showrunners, it has to do with Sean's new characterization as a cop in "Boston Blue" and the desire to bring "different energy" to the series.

"Blue Bloods" was canceled by CBS back in 2023 ahead of its 14th season, prompting confusion and controversy because the series was still a big ratings hit. Regardless, the Reagan family will not be returning — at least not as a whole. Instead, Wahlberg's Danny Reagan will head north in "Boston Blue," which had its premiere on October 17, 2025. Notably absent from the cast, however, is Andrew Terraciano, who since 2010 had a recurring role on the show as Sean Reagan, the youngest of Danny Reagan's two sons. Appearing alongside his real-life brother Tony Terraciano, who played older brother Jack Reagan, Terraciano stayed with "Blue Bloods" throughout his run, appearing in 292 episodes and essentially growing up on-screen.

With that in mind, it's surely a risk to recast a character who had become so familiar to viewers. But that's what "Boston Blue" co-creators Brandon Margolis and Brandon Sonnier did. Mika Amonsen plays Sean in the spin-off, with the character joining the Boston PD and embarking on his own law enforcement career — all of which apparently required a new actor.

There have been controversial recastings before, but much like the "Blue Bloods" cancellation, Andrew Terraciano being replaced is just sort of confusing, even after you hear the explanation. "Boston Blue" co-creator Brandon Margolis spoke to TVLine, revealing that while he and fellow creator Brandon Sonnier "love[d] Andrew's work from the original series," they had to switch things up for this new story. "Once we realized we were telling a very new chapter in this character's life, creatively it made sense to find an actor with a different energy," said Margolis, who added that Mika Amonsen's performance echoed "Sean's vulnerabilities" from the original show but that he brought "a youthful energy that takes the character in a new direction."

The "Boston Blue" premiere made no secret of this major recasting, opening on Mika Amonsen's newly instated Boston P.D. officer Sean Reagan as he patrols alongside fellow rookie Jonah Silver (Marcus Scribner). Things quickly take a dramatic turn when the pair comes across a burning building, and Sean is caught in an explosion, which sends him to the hospital. It's this unfortunate event that brings Danny Reagan to Beantown and provides the groundwork for the entire spin-off.

Asked about their decision to open the pilot episode on the newly recast Sean, Margolis admitted that it was a "gamble" but claimed the story required the audience to "love him right away" and that "opening with that heroic moment that has a tragic end was the right way to pull Danny Reagan into this world."

Brandon Margolis expanded on the new storytelling requirements of "Boston Blue" in an interview with Variety, where he spoke about Sean Reagan becoming a cop being a major factor in his and the producers' decision to recast the character. "It's not just Sean who has joined the family business as it were and is turning over this new leaf," he said. "It's also now Danny Reagan is the father to a cop, which he's never been before. So there are all these new storytelling lanes to explore." According to Margolis, this provided an "organic opportunity" for "a new energy" that could help emphasize the change. "It is the same character, it's the same relationship," he added. "But it's also completely different. We love what Mika has brought to Sean: this vulnerability, yet this desire to prove himself. It's just been great to watch it come to life."

It's not entirely clear why Sean's becoming a cop necessitated a new actor, but it seems the showrunners were convinced it did, regardless. There may well be several other behind-the-scenes reasons for the recasting, but thus far, there's been no confirmation of anything. Meanwhile, Andrew Terraciano has yet to directly address the recasting. The last "Blue Bloods"-related update from the actor came in December 2024 when he made an Instagram post celebrating the series finale. Otherwise, the actor has been busy graduating from New York University in 2025, with his former co-star Donnie Wahlberg congratulating him on a separate Instagram post.

Elsewhere, it seems Tom Selleck is eager to team up with "Yellowstone" creator Taylor Sheridan, so if that ever comes to fruition, perhaps Terraciano can come on board as a similarly jilted ex-"Blue Bloods" actor.