Looks like MT-Blacklist wasn't working. Unfortunately, I'm about to go on a trip and can't spend a lot of time researching/implementing a replacement, but the one I found here looks promising. The new tactic is that a hidden value needs to be passed to the form handler in addition to the standard MT comment form data. Spambots that work by sending the info directly and bypassing the actual comment form should be thwarted. Hopefully.
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