Yeah, what Luger said. If you haven't watched a class yourself yet, do it. If they don't let you watch the class, forget it right there, if you ask me.
As for the instructor's weight; Sometimes people don't have the talent to do something themselves, but have the understanding to teach it and to teach it well. His weight is definitely a legit concern, but I wouldn't rule him or his school out because of that. Just pay attention when you watch a class, and really study the instruction he gives. If the instruction is there and the guy can turn out strong grapplers, then his own weight is irrelevant. If the instruction isn't there, then the school may not be worth your interest.
As for a blue belt teaching a class; Belt rankings in any martial art are to be taken with a grain of salt, if you ask me. Consider them, respect them, but keep in the back of your mind that a belt can lie as much as the person wearing it. An illustration: I could outfight some of the far higher ranked guys in my gung fu class, because I could better put into application the basics than they could. But because I couldn't perform some of the more advanced techniques that they could, I didn't have their rank. Not to mention, I didn't pursue rank like some people did/do. It's very possible the girl didn't care much for rank and only started after it after she had been training for some time. I'm not trying to entirely stick up for this blue belt teaching a class though - because just as much as belts can lie about someone's fighting prowess, fighting prowess can lie about someone's teaching ability. Just saying, consider both sides, if you haven't already.
I don't know of that guys' teacher as I'm really not knowledgable about submissions/notable BJJ guys. Sorry I can't be of more help though.