Personally, I'm fine with mutations being imported from other species. I have plenty of Ultra lurking in my collection. Of course, we have no idea if that is the case here or not. However, Florian is a professional breeder who does this for a living (for profit). Eventually he will sell some of his melanistic corns, and the price will no doubt be significantly affected by whether or not the community thinks his snakes are hybrids. IMO, being transparent, and sharing photos of the parents and siblings can only help him establish his case, and I hope he decides to do so.