I am so sorry to hear about your loss... It sounds like he lived a long happy life with you, though. 8 years is no drop in the bucket. I have read that most captive bread frogs live about 7 years minimum, so he for sure lived a good life. I have never heard of a black fungus on tree frogs! SO strange. I wonder if it started growing after he died or if it may have been in his body to begin with and over took him after he died? It makes me curious and I am going to look into it myself. I am deeply sorry for the loss of your little friend. It does not sound like anything you have done, I mean if it was that simple I don't think he would have lived 8 happy years. If I find any info on the black fungus I will post it here for you. Maybe it will help shed some light on what happened. If you have any other frogs or other fragile pets, I would clean up his home A.S.A.P. so it does not spread just to be safe. Remember the good time you had with him and Best Wishes!