Black jellyfish?

Has anyone ever seen black jellyfish for sale before? The closest thing I found online was called a blue bubber jellyfish. I saw 3 black ones at a saltwater fish store in Dayton Ohio today. Tried and tried to find specs on them but no go. They looked awesome and about the size of a half dollar. I'm sure they wouldn't go good in my fish tank but I was curious of how to keep them and care level. They weren't bad priced either at 19.99 each. Wish I would have taken pics of them