If you give someone a blow job, make out with him, and then he goes down on you… it will not make you pregnant.
I suppose that if you held the semen in your mouth, passed it to his mouth, and then he tried to spit it up into you like a whale shooting water from its blowhole, there might be some chance, but…. you’d really have to work at it.
Explanation for why you can not get pregnant from a chain of oral exchange:
If a guy comes in your mouth, which has its own risks, you will most likely swallow the semen or spit it out. Then, when you kiss him, most of it will be gone already. When he goes down on you, he will have presumably swallowed at least once. There’s not going to be living sperm in his mouth to get inside of you. Even if you passed your semen to his mouth and he swallowed it you’d be fine.