My own stupidity, that is.
TL;DR: Leonard is just fine. Now the rest...
I called the vet yesterday about the black splotches I saw in Leonard's mouth and she said that although they didn't sound worrisome considering the rest of his health indicators (eating vigorously, no signs of illness, no "oozing", etc.), but she couldn't say for sure if she hadn't seen him so I took him in.
We got his mouth open and... well, nothing. Everything is pink and clean, and the only black pigmentation is right around his esophagus and where his tongue emerges. What I saw was either a shadow, or some remains from his lunch that day... and we're leaning more toward shadows. My genius knows no bounds.
So I feel like a bit of an idiot, obviously, but at least we have had a GOOD look into his mouth and he is fine. Better safe than sorry, even with a vet bill for it.