The black key from netherrealm does pretty much nothing. and The gold key you get from the old lady opens her hut in the village. It's the hut with every colored koin in it.
Well hope that hellped.

The black key in the Netherealm is a little peculiar.
Think about it this way.
I would normally dismisses this kind of thing if it involved some sort of quest (Raiden's staff seems to be just an Easter Egg that holds no significance according to Fight Night).
But what about this key? You didn't need to do anything to attain it. It was hidden on the dead body. Why put something like that in the game for absolutely no reason if it's useless?
Makes me wonder what purposes it serves.

kinda like the ninja star. you get it for waking up the kid sleeping near bo rai cho's academy in the village in earthrealm. so far, i have found absolutely no use for it. i have also found multiple Shira-Ryu clan member masks. i have only found 1 use for any of them and that is to give it to scorpion in the chaos realm but you can only give him one.