Thank you so much for saying this and making that distinction. I've never seen this worded so concretely before. I really, really hope OP takes this into consideration.

Maybe it's because of the term "chubby chaser" that it seems like less of a physical/sexual preference and more of an outright objectification. Sure, there are some couples who don't start off physically attracted to one another and that develops after they get to know each other. There probably even more couples who are physically attracted to one another first.

I have a physical preference for fat people. This is not a hard rule. I've been with all kinds of body types, men and women, fat, thin, and strong-man-keg-lift-nail-bending personal trainer. I absolutely love my fiance's body--it's literally my ideal. To quote a comment I made earlier today in another /r/relationships post: "my fiance is the most gorgeous man whose presence I've ever been graced with". But it's not the reason I got with him, it's not why I'm still with him, I sure as shit didn't get engaged to him because he's fat, I would stay with him and be just as happy and attracted to him if he lost weight, I would absolutely never consider being unfaithful over weightloss, and I like to think he knows everything mentioned in this run-on sentence.

OP, your boyfriend loves you and he does not seem to be objectifying you from what you've said. If he can't be supportive in the long run, that's one thing, but I think it would be good to try to reframe this before you make any major decisions. This is like any other physical preference a person might have. And these concerns here:

But what if Blake can't handle me being thinner? What if he leaves for a woman that fulfills his sexual desires? Will he be unhappy with me losing weight? Will he try to sabotage me?

Those are pretty much the feelings he's been battling with lately and you knew they were silly. You knew he was wrong to worry about those things. Give him some credit.