why the blackbyte is cheaper than a byte in the slack?
Probably because byte is incentivised by the rules of the distribution and easily exchanged on Cryptox. Moreover, blackbytes do not have an actual use currently and holding bytes will grant blackbytes in the distribution, so, some people get rid of them ...
Blackbytes could be the natural asset to be used in a double deposit escrow trade: that will force both parties to put a deposit (in Blackbytes) and only they can sign off on it. If they both cheat they both lose their deposit in blackbytes