My Axolotl is darker on one side of his belly?

I just got an axolotl 5 days ago, he/she is a juvenile golden albino. His stomach i can see inside on one side is dark like dark grey almost black but i noticed the other side is white/clearish. Plus his gills are curled forward and there was a ton of this black stuff in the tank like poop but before he was eating pellets and he pooped brown stuff and hes eating bloodworms now and he loves them and im assuming this black stuff is poop but could it be vomit or something else. There were snails in the tank before we got him but i removed them all and haven't seen any since and we have a sand bottom with lots of large rocks bigger then him. Nothing big enough to swallow though. His belly seems proportionate and symmetrical other then color. The temp right now is about 19 Celsius and im going to try to get it lower but right now i cant do much. I checked the water and nitrate and ammonia are normal. Hes been eating fine and likes to swim around the tank whenever someone comes by. He did swim directly into the glass a few times and bonked his nose but it never seemed to faze him.

Your axolotl sounds healthy and normal to me. All axolotls have slightly transparent skin when they are very young, especially any albino coloration such as yours. The dark spot is simply his stomach, nothing to worry about at all.

If you are seeing blackish Brown pellet or log like droppings in his tank that is poop.