Marvel Comics for Black Widow
The key first appearances for the upcoming Black Widow movie are Avengers 43, 195, and 196 for the villains. Avengers 43 is the first appearance of the Red Guardian, who will be played by the actor that played Hopper in Stranger Things. Avengers 195 and 196 are the cameo and first full appearance of the Task Master. It should be obvious that the biggest first appearance will be that of Black Widow in Tales of Suspense 52. It is also possible that the Tony Stark A.I. may make an appearance at some point or it could just be a flashback with Robert Downey Jr.