I've never seen it in chameleons but I did have this happen with some leopard geckos years ago. I put them in together to breed and came back 10 minutes later to a bin speckled with blood everywhere. I couldn't find any injuries and they both looked fine but I was at the vet's with them the next morning. We checked the mouths, the vents, toes, eyes, etc., and nothing.
I emailed a few large breeders and I did get a couple replies back saying that they'd seen the same thing from at least one or two virgin males. It was never a problem or a health issue, nor did it ever happen again to the same male after that first time.
So I don't know, it may have been a little nick on a hemipene, or a ruptured blood vessel, etc. But keep an eye on him. If he goes on like nothing then great, but go to the vet if he keeps looking stressed/still has sunken eyes.