This is only a slim chance, but it could also be one egg that never got laid. Un-laid eggs need to come out right away.
I've never heard of an iguana having a bezoar. They are created to feed on very fiberous leaves. But I've heard a lot about iguanas who have swallowed a foreign object. It could be a peach pit or a marble, or anything he can swallow. Igs are famous for doing that, which leads to surgery. My favorite iguana began to get sick for no reason, an finally had an x-ray. It showed a dime stuck on his stomach wall. After surgery he acted like he never had the surgery. Dime out, zinc poisoning stopped.
I don't have direct experience with black heads, though I've heard of them. Cold or stressed iguanas often turn dark. The other thing it could be is he is changing the color of his skin so it will absorb heat better. I've read on this site about iguana that turn blackish when they feel the warmth and the light of the outdoor sun. It's a response that lets them gain more heat. So double check the temperatures in his cage.
I'm very interested in your iguana's diagnosis. I hope you have a good exotics vet.
It could also be a matter of a buildup of old sheds. Tubbing him in 88F water often cures that problem. Remember that 80F swimming pool water feels cool to a human.
Dehydration may also be the culprit. Iguanas need to take in moisture with their food, and also in the wild they drink from moving water. When they get dehydrated, several other diseases spring up.