Now we have a bot that sells blackbytes for bytes. The sale is through a conditional payment smart contract, so the bot can't scam you.

The rate is fixed, and it is a unidirectional sale only, the bot doesn't offer the reverse exchange.
The bot is already in the Bot Store.
Source code:
https://github.com/byteball/conditional-token-sale.
You can easily change a few parameters and use it to sell your tokens.