If you're doing agent research, remember that you want an agent who wants to work with you, who wants to rep you longer term, and who will be comfortable advising you on difficult career decisions, including PR.
I keep telling people: The question about an agent isn't "Is he so bad I should cross him off my list?" It's "Is he good enough to put on my list?"
Go to a bookstore. Find books like yours. Find out who represented them. Get those agents' guidelines. Follow them to the letter.
Meanwhile, write another book.
Be professional and polite, but it's about your interests--that's what you're asking an agent to represent.
I don't think there are blacklists. It would take far, far too much work, for one. It's hard enough for agents to keep track of queries as it is. Plus, any agent who decided to create one would be hurting themselves more than anyone else, for the reasons AW Admin mentioned. An agent who doesn't sell isn't an agent for long, nor an agent you want.
There are a few agents--and more than a few writers and journalists--who, IMHO, spend
way too much time on Twitter considering 99.9% of the business we're in does not happen on that app. Everyone's got their own priorities and approaches, of course. We're all adults here. But ask yourself: do you want your agent fighting Twitter wars on your behalf or do you want your agent selling books? Different people will rightly have different priorities, but you're entirely free to choose who you query, just as anyone, including an agent, is free to choose who they want to or don't want to work with.
I sympathize with the grad school thing, and have had the same thing happen. You know how much it ended up mattering? Not a jot. The person who did the bad-mouthing is long on to other things and nobody cares about people who gossip anyway. While I can't know why I may or may not have gotten certain jobs I've applied for, I'm better off not knowing. If someone really does care about petty stuff, I don't care to work with them.
Beware reading anything into rejection. It's part and parcel of being a writer, and there's only madness in trying to read the tea leaves.