Black Widow (real name Natalia Romanov or also known as Natasha Romanoff), is one of Marvel Comics’ earliest heroes. She actually didn’t start as a hero, though. In her first appearance way back in 1964, she was presented as a Soviet spy assigned to assassinate a defector that was working with Iron Man (Tony Stark) to develop new armor. Through the course of the next couple of years, though, Natasha fell in love with Hawkeye, fought and then made friends with the Avengers, and had several adventures in various comics. For the first five years or so, she wore a grey-ish/black costume with an eye mask. The outfit included a small cape. By 1970, though, Marvel had ditched a lot of their characters’ older costumes to reflect more modern fashion sensibilities.
Marvel artist John Romita, Sr. was asked to re-imagine Black Widow in a new costume. She was going to feature prominently with solo stories in a new title called Amazing Adventures, and Marvel wanted to give you some additional publicity. Romita designed a costume based on a Golden Age character named Miss Fury. She also looked quite a bit like Catwoman in a sleek black bodysuit with a full head mask. Romita opted to drop the mask entirely to expose her face and showcase her long red hair for which she is distinctly known, and provided a low-slung decorative belt with gadgetry. So, in 1970, a fabulous and famous look was born.
In order to introduce her new costume and give her additional visibility, she was re-introduced through the powerhouse Amazing Spider-man title. In issue #86, Black Widow spots Spider-man after a battle and determines that she needs to find out what makes him tick. She’s looking to get back into the hero game after dealing with some personal issues and in order to make the break, she decides to ditch the old costume, create a simplified new one that’s also “more in keeping with the swingy seventies,” and add some powerful new fighting features.
Here’s the story from Amazing Spider-man #86. Notice something interesting? The shadow of Black Widow on the cover appears to have a jacket. Perhaps it was done before the final decision was made on the costume she ended up wearing inside the comic.
There’s a really good post with some more details that I recommend. Take a look at Agent of S.T.Y.L.E. – Black Widow Keeps It Classy