The only way I see something like this working is if communities had tags. Each community would select from a list of pre-determined tags based on the content posted there. Then, at any time, users can then block tags as they see fit.

There would have to be some incentive to encourage communities to add the tags once they are implemented. The most fair way I can think of is to have a “None” tag that every community gets by default. Instances, or even apps can choose to hide untagged communities by default, and users can choose to hide them too. The incentive would come from the potential loss of visibility by not tagging your community.