There are no rules for blacklisting someone. You can do it as much as you want, freely, for any reasonn or none, without any justification.
It has noting to do with ToS. It cannot have anything to do with banning people.


On the other hand, reporting someone has everything to do with ToS.
It has already been suggested time and again to have a temporary auto-ban on people who have been reported by [instert a fixed number] people, to remove the probable offender temporary from the server, and give time to SE to check to make sure this person indeed violates the ToS.

Considering that the report function is so impractical that many people (me included) have stopped reporting offenders, an easier way to report someone would need to be added into the game, but the auto-ban could have some use regardless.

Considering that some claim they have been reporting bot users for months without any result, it is possible that SE is too inneficient to investigate reported people in a timely manner, which would make the temporary auto-ban impossible to implement (the "temporary ban" should be short, because if the person happens to be innocent, they should not be kept from the game they are paying for).