I can see uses for blacklists as there will be people who create lots of accounts to spam Hive. So far that has not been too much of a problem, but it is likely. It's not feasible for every user to mute every spam account and so we will need this. Being able to choose which blacklists we use would be great. Some people may want to see troll comments for whatever reasons. Bring it on!
The issue you're getting at above is actually a slightly different one, which is being able to follow someone's mute list and have it add to the user's muting function.
We discussed that idea in our office as well and I think it would also be a nice feature to add. It would be also be a relatively easy feature to add. But the immediate need was to find a way to speed up the performance of API calls being impacted by the blacklist processing.
Being able to subscribe to someone else's "mute list" would be a cool option.