If the descentralization of social networks continue, we will have to prepare for the eventual rise of the instances wars, where people will start to fight about which instance is better and which one is weird to be in and so on, but that's for the future of us all.
One thing that is feasible is for established instances to give votes from new instances a lower weight. So, no blacklisting, but until they have been around for a little while to be able to calculate that their activity corresponds to their size and that nothing is off, upvotes and dowvotes could be ignored or given a lower weight.