Entity UUID and uniqueness We have a multisite install that will have a few shared blocks. As we spin up more sites we'd like to just take the yml file associated with the block and add it to the config directory of the new site.  The issue though is the UUID.  Do these need to be unique across the entire install or just within a sub-site?
A multisite is really just multiple sites that happen to share the same code base. Unless you do crazy things with sharing prefixes (which I would really, really recommend against), the fact that you are sharing code is completely irrelvant to the running site/configuration.

So, there is no such thing as sub-sites. Just sites. And it doesn't matter if the have the same UUID's in different sites.