Are you really sure Lord Krishna was not actually blue? Do a search for “Blue Fugates”. Some people have methemoglobinemia, a genetic condition where the blood carries less-than-normal amounts of oxygen, making the blood appear blue. They lived healthy lives. There are some modern people who have/had argyria is caused by the ingestion of silver, usually for medicinal purposes. I have seen two blue/blue-gray people in Tucson, Arizona when I lived there.
Dear Elaine, I doubt that there is any absolute way to say what color Krishna’s skin might have been. Most likely, it is a poetic way of saying he was filled with great spiritual light — and blue, of course is a very “spiritual” color, being, as Yogananda says, one of the colors of the spiritual eye (along with white and gold). Or this may be a reference to his aura, rather than his actual skin color.
The scientific information you have offered is interesting, but I don’t know that it would be necessary to say that the Lord Krishna might have had one of these conditions and therefore had blue skin. If there was some sort of spiritual reason for him to have radiated blue light from his body, then no “logical” explanation would be needed. It would simply have been part of the very creative, divine lila (the divine play of God) for us to enjoy.