Among the more persistent items of writer mythology--agents are likely to steal your work, publishers aren't interested in first-time authors, you can't get an agent unless you're already published--is the notion of blacklisting. Many people fear that there's some sort of master reference list that agents and editors use to identify troublesome writers, or that agents or editors can turn a writer into an untouchable by passing his or her name around among their colleagues.
This idea is actively encouraged by scammers and other disreputables. Writer Beware has documented many instances in which an agent or publisher has responded to complaints, questions, or demands with a threat of blacklisting. "You'd better back off," the writer is told, "or I'll see that you're blacklisted throughout the industry." Or, "You know about the blacklist, don't you? If you get on it, no one in publishing will ever deal with you again."
Don't believe it. It's nothing more than an intimidation tactic intended to frighten you or shut you up. There's no such thing as a "master blacklist." Period. Nor can an agent or editor singlehandedly blacken a writer's name throughout the industry--nor is it likely that they'd care enough to bother. Keep in mind also that the people who typically threaten blacklisting--the fee-chargers, the amateurs--are not actually part of the publishing industry, and haven't the connections or the credibility to blacklist anyone, even if they wanted to.
The ease of searching on the Internet has also sparked fears of blacklisting. Suppose you ask a question about an agent's background on a writers' message board. Suppose the agent does a websearch and finds your comment. Will the agent be pissed off--especially if you have something critical to say? Will she tell her colleagues? Will your name become mud?
Frankly, I doubt it. Certainly there are agents who do websearches on potential clients, and are influenced by what they find. But it seems unlikely that they'd bother to circulate someone's name, even if they learned something that displeased them. Why would they care that much? Why would anyone else? Also, for every agent who has the time and interest to do that kind of research, there've got to be as many, if not more, who don't. Obviously you need to use your common sense. If you post something snarky about the agent who just requested an exclusive on your first three chapters, it may come back to bite you. If you behave in an obnoxious way--sending nasty emails to agents who reject you, for instance--you may become an item of gossip. But systematic blacklisting is not something that should be high on your worry list.
There's a larger issue here, of course, and that's the fact that nothing is really private on the Internet. Just as job-hunting college students have found themselves embarrassed when potential employers discover their MySpace pages, writers need to remember that blog entries, message board posts, and Usenet participation are public discourse. If you take responsibility for your words, and say nothing online that you aren't willing for everyone in the world to see--including the agent you queried last week--you won't need to worry about who's doing a websearch on you, or what they might learn.