I really don't see how Mimi is supposed to look anything like Blossom. I guess she just inherited more looks from her father, but there just seems to be almost no resemblance to her mother.
Anyways, speculation time! Alright, so we know that in PPGD time, Blossom and Dexter are set to hook up. We know that Blossom pursued a career in science and looks even more like Dexter with the glasses and lab coat. And since it's only logical for people in a long-term relationship to grow more similar...I think we can only assume they got serious eventually. But obviously Him is Mimi's father, and I don't see any reason Blossom would ditch Dexter to go romping with pseudo-Satan.
Possibility A) Him somehow disguised himself as Dexter and/or somehow tricked Blossom into making the worst mistake of her life. Given that pretty much every otherworldly being thus far is capable of some degree of shape-shifting, I guess it could happen.
Possibility B) Dexter died, Him cut a deal with Blossom to see him again and/or resurrect him in exchange for...um...Favors. Him pulls an ironic twist to the deal and Blossom is left heartbroken, vengeful, and pregnant with a half-demon.
Anyways, we've got no idea who the traitor is yet. One random thread of possibility dangling from my brain at this point is that maybe Dexter was the traitor, albeit perhaps not knowingly. We still don't know what happened to him.
One last aside: Shouldn't Raven be noticeably more mature than Blossom, not the other way around? Then again, the Bleed-verse timelines are a friggin' nightmare.