Either common or comet goldfish, looks like. Their colors can be unstable, and black is the least stable color in a GF.
I have some that change like that -- some that never change and some that change only once. They may not be very attractive at the moment, but give them a chance as you never know what you'll end up with.
I had one that stayed black for over a year, then when it started changing I didn't know it was the same fish -- thought that "Mystery" had died or been eaten. That was last year, when he went to some ugly white, light orange/mid-orange mottled color. This year he's still changing, and is a mix of a creamy white and some oranges -- for some reason, his color reminds me of a mixed ice cream cone -- and he's actually a pretty fish! No sign at all of the black.
I have a couple of babies (spawned this spring) that are starting to color -- they look just like yours.