Blocking could be checked against two tables (or even a table array). The first one would be per-wiki, while the second one would be shared between wikis.
Then, how would it be configured? *Should any wikipedia admin be able to (un)block anyone? *Should it be only done via meta / stewards? *How would global-blocking be locally handled? Should local project have a local-whitelist? *An special page for it or integrated in the specialblock (a new checkmark for example)? *If we have blocks on both tables, with different expire time, which should apply?