I'm not the most hardcore Black Widow fan...I've only become a fan of hers in the last couple of years, so I didn't know about the hourglass logo. That's pretty cool. Joss using his comic knowledge I guess
As noted it's been standard on her for decades.
Examples for you and anyone else perhaps not as familiar with Natasha as the Black Widow from the comics.
Take note of her belt. You'll also notice her wrist gauntlets are nearly identical in design to what we've seen in IM2 and the Avengers publicity photos to date EXCEPT for the gold coloring.
Personally I'm hoping Tony touches those up for her at some point.
An actual female Black Widow spider with her hourglass red marking.