Thats the problem. It would be very difficult to get a new instance off the ground unless you were an insider or had inside connections. If you have a cabal of existing admins acting as gate keepers you could keep outsiders from abusing the system easily, but you are also walking right back into the centralized control federation is supposed to prevent.

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.

Well non-federated forums can grow by word of mouth and similar. Being federated does lower the barrier of entry for interacting but it's still possible to visit the instance the old fashioned way. You probably still need to rely mostly on word of mouth anyway, even if you are federated.

Yes, age alone shouldn't lead to getting blacklisted. But if an instance is two days old, already 50+ accounts from there were banned on your instance for being bots and besides that there was no real contributions coming from that place, this might be a candidate for auto-blacklisting.