Modifying the launcher allows you to join blacklisted servers

Changing the libraries Minecraft is using (by for example modifying the version json files) allows you to join any blacklisted server

It might be good if additionally blacklisted servers are blocked from getting the shared secret (either by checking the IP from the client or blocking the server itself) from the Mojang API (see http://wiki.vg/Protocol_Encryption#Authentication) so no users can join (except the server is running on offline mode, something that is not desirable to most servers).