Blackjack has barricaded himself inside a cave down in the canyon, near Mesa Verde. He had been running from Alexandra, who's been on his trail for days, and he didn't realize the canyon didn't go any further. He claims to have a minigun and hopes Alexandra will eventually give up and go away.
Alexandra has set a small camp behind a 3-foot high wall of rocks. She caught sight of Blackjack several days before this encounter and chased him here. Blackjack is worthless to her dead, but she doesn't have the faintest idea how to get him out of the cave without facing his minigun. She's hoping she can starve him out, but with his super mutant endurance, that doesn't seem too likely. His "minigun" is actually just a bunch of big sticks and pipes tied together in a bundle and smeared with charcoal to dye them black. He's also got a big pack of supplies, so he'll be able to hang out in the cave for quite some time.
Resolve the Alexandra-Blackjack situation - Blackjack is barricaded, hiding out from Alexandra. He doesn't want to have to kill her, but he doesn't want to shot, either (he's unaware that Alexandra needs him alive). If the player character gets Alexandra off Blackjack's back, he'll head over to Hoover Dam to go find some card games, and Alexandra, if still alive, will head over to Fort Abandon to hang out. If the PC helps Alexandra capture Blackjack, she'll take him to Denver and give him to the Caesar's Legion slavers before returning to Fort Abandon.
Blackjack has not appeared nor is he mentioned in any published game. He was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if he will appear in any future Fallout game.