Marvel Studios President Kevin Feige says that developmental work on a standalone "Black Widow" film is still underway, just don't expect it too soon:
"I think the idea would be great. We've got various outlines and ideas of where to take that. As Scarlett already said, there's a big element that explores her back story in an upcoming Marvel feature. The question really is, 'When will we take her out of the ensemble and have her do her own thing?' As you saw in ['Captain America: The Winter Soldier'], as you'll see in 'Avengers: Age of Ultron,' she's really the key to so much of the plot development."
"Captain America: The Winter Soldier" MINOR SPOILERS AHEAD
Meanwhile, actor Sebastian Stan tells Newsarama that his contract with Marvel lasts for a whopping nine films, but as an actor he's not "told anything" and so has no idea what films he'll be incorporated into.
That's three more films than the standard six-film contract all the others are signed up to, bar Samuel L. Jackson who signed a similar nine-picture deal with Marvel when he came onboard as Nick Fury.
Source: Coming Soon