Black is almost impossible to keep on almost all goldfish that are not calico or black moors. I have had a few bicolor (orange and black) ranchu and telescope who all lost all or effectively all their black. I was fortunate enough to come across a nice tricolor telescope who is not just an orange and white telescope.
I personally don't buy any online, they are too sensitive. I know the ones that make it from breeder, to wholesaler, to a LFS are hardy enough to not worry about.