Well, by the time I got up this morning I changed my mind about "watchful waiting" for Bob, and he went to the vet this afternoon.
At first the doc said he agreed with me that it looked like a tumor, but he felt it and gave Bob a shave to get a better look, then said it seemed more like a wound. Since Bob lives alone and doesn't get playtime with the others, and there aren't any sharp edges on his cage, a wound seems weird...but better than a tumor, at least. Anyway, the vet decided to scrape the dark tissue, and darned if it didn't come off...making me feel like an idiot for bringing Bob in because he was dirty.
No, the vet said, there's still something going on on the skin. We've got Nolvasan and a topical antibiotic to treat it for a week, and see what happens.
It still might be a tumor...but maybe it isn't (all fingers crossed!), and now that he's shaved, keeping an eye on it will be easier.