Getting 1 of X duties randomly constitutes a roulette. Nothing about blocking duties would make the roulettes any less of a roulette with the possible exception of the case where there is only 1 choice remaining.
In any case I don't see the downside. A duty blacklist is essentially designed to solve the issue with Alliance Raids where there are seemingly two strong preference groups. One group wants to get CT and AFK, the other wants to get anything else and play their selected class. Let people select there preference and nothing is lost, and then in addition there could be more players willing to queue and help fill in for duties if they aren't dissuaded from signing up in the first place out of dread from getting something they dislike.
For the other roulettes I don't see as clear a case for the benefits of duty blacklisting, but the downsides should be minimal, if they exist at all. There is also flexibility in how to approach blocked duties. A true blacklist would make it impossible to get a selected duty even if it was the only thing players were queuing for. Conversely a less strict graylist would try very hard to avoid queuing into something but give up if no other options were available. That would at least solve the problem of getting 4 level 100's in Satasha.
