To explain (and there may be spoilers involved), Black Widow was a part of the Ultimates, but in The Ultimates 2 graphic novel, she betrayed the team to the bad guys and, in the end, was killed by Hawkeye in revenge (his family being killed earlier in it). Considering everybody remembers and likes Black Widow, for here, I've arranged it so that Natasha Romanov was still alive, but captive, and it was another Black Widow impersonating her (in fact, there was a Black Widow named Yelena Belova in the comics, but she was blond and bad).