Fallout 4 created by Bethesda Game Studios.
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Prerequisites: Complete Butcher’s Bill
Trigger: Speak to Doctor Carrington
Reward: 170 Bottlecaps, 300 XP

- After completing Butcher’s Bill, go have a nap or do something else for a while, then return to Railroad HQ and speak to Doctor Carrington again. He’ll have a new quest for you - namely, this one. 

- As before, Carrington will set up a mailbox dead drop called Blackbird Report for you to retrieve. There’s no telling where it will be, in fair neighbourhood or foul, so be ready for anything. (I got lucky and received mine right outside Hangman’s Alley, where I have a settlement. Kinda like standard mail service, really.) Check out the Report for details on your next destination.

- The rest of this quest is really up to you, as Butcher’s Bill 2 leads you to a random location to find Blackbird… or, uh, the remains of Blackbird. He’s located somewhere in the world, and you’ll have to go looking for him, based on the waypoint you get. This can take you any number of places - for example, I wound up in a little Drug Den near the USS Constitution, while I read that someone else wound up in the Federal Ration Stockpile, which is waaaaay on the other side of the map. Either way, expect baddies.

- Return to Carrington to get your reward, per usual with dead drops. This will complete the mission, and if you speak to P.A.M. afterward she’ll give you a new mission: Mercer Safehouse.