Not sulfur yellow. Diarrhea. So, considering that it looked pretty normal as far as I could tell...maybe it wasn't BH? I
So very sorry for your loss, I know how heartbreaking it is to lose one.
If I were you I would do my own necropsy. If it was blackhead, coccidiosis, or roundworms it will be obvious. Blackhead poop is not always yellow.
Other things to look for are things he could have eaten like sharp pieces of plastic or metal, that's pretty easy to spot too.