I've encountered girls who "don't" do certain things and I've talked with Johnny Soporno about that issue as well. Basically the way I see it (note, I'm not quoting Johnny, as I can't recall precisely what he said to me), there's two ways to handle the issue; either you confront it and deal with it, or you ignore it and hope it goes away.
The key to the first method is to ask the girl why she doesn't do whatever it is, or what it is that she doesn't like about it, whether she has done it before, or whatever else question you can use to find out some info about what her problem with the issue is. Then, you present some reasoning that allows her to excuse the issue and no longer thinks that her reasoning is valid for prohibiting her from doing it. In the case of a blowjob, you ask what she doesn't like about giving them (remember to be as unconfrontational as possible during this process) and then you ask if she's ever given one before if you are unsure. If she hasn't and she says that they're gross, then you say that she has no experience to judge it off of and although you are ok with it if she isn't interested in giving you head, you let her know that you really enjoy the experience and that if she was open to seeing whether or not she really doesn't like them, then you would be happy to give it a try with her and go through it with her. Remember, this is all about creating a feeling that you are there to make her feel safe and comfortable during the whole thing, as well as super sexy.
The second method is like one of the other posters mentioned. You basically pretend they never said that they don't do such and such and then you continue getting them turned on and escalating things to the point where they are so horny that they shed their inhibitions and do whatever it was. The key to this tactic is having skill with seducing and pleasing women very well, so if you aren't very good in bed, this is going to be a harder route to take, BUT you can always get better with practice and eventually you can crack her.
Failing all that, there's Chief's "go find a new chick" method.
