There is no blacklist on Hive. There is a list of accounts that are not being airdropped with free coins on the new chain.

Why this list is only short before the HF aviable?

Inherent time pressure. It was literally just finished hours ago. Some people (not me and I won't take credit where it is not deserved) have been working >16 hours a day to pull this off.

There is a mechanism to adjust the list or correct errors (apparently there is one) after the fact, with the consent of Hive stakeholders.