They are for anonymous transactions.  You can only send blackbytes to a linked wallet.
You can't trade them on major exchanges like Bittrex, but you can trade them on the Byteball slack under trading_blackbyte.

You should read this article on how to do it, so that you're safe doing these peer-to-peer trades using Byteball's conditional payments.

I should warn you though, blackbytes aren't worth much at all currently.  You might be better holding them long-term until they're worth something significant relative to bytes.