same thing happened to my sister's tamagotchi. It has water in the tama! but i know the problem! just let it dry for 1 day and see what happens! *BAD NEWS* you might have to RESET ...IF... the tamagotchi is in shape!
If the crack in the screen ( thats probly the reason of the black splootches (lol) ) gets really bad you might unfortuately throw the poor tama away or give it a proper funeral.
I've never had that happen to me before. Either it's cracked or it's well, poop. You should reset and if that doesn't work, maybe get a new battery and if that doesn't work then I guess you should get a different one.
One of mine went all black, like all it's lights were on at once. I just took the battery out and put it back in and my character came back without losing any points or anything.