Well I have a question if they do stay together, do you think this will help her career (I guess a la Aniston and Pitt or Paltrow and Pitt) or will it hurt it, like it hurt Katie Holmes or Nicole Kidman who said that her agents advised her against marrying Tom Cruise since she would become known as Mrs. Tom Cruise?
I think it depends. If Leo treats this relationship like his past ones (no red carpet appearances, no acknowledgment of the relationship, little PDA), I don't see how Blake can benefit.
Dating Leo helped Bar and Gisele because they weren't regular features of Us Weekly or known to the public at that point. So his simple association was able to raise their profile, get their name out there, and help their careers (although Gisele was already successful, she wasn't really a supermodel to the public yet).
But in Blake's case, just being known as Leo's girl isn't going to do much for her career. Her public profile is already high, thanks to her PR, Vogue, Chanel, etc. And it's not like she needs him to introduce her to producers because she already has family connections, Weinstein, and a great team who was able to get her auditions for Gravity, Gatsby, and Dark Knight (she didn't get them, but Leo can't really help that).
An officially acknowledged relationship wouldn't hurt, but the only thing that will actually make her a genuine movie star is if she gets a lead role in a great (critical or commercial) film. And she already had all the connections needed for that; she just lacked the talent to back it up. Leo's not going to be able to help with that unless he gives her acting lessons.
At the same time, I don't think the relationship will hurt her career because it's not like she shies away from being media attention. And it's not like dating Leo has made everyone stopped talking about her actual acting talent because no one was doing that in the first place.