Which is what makes this so funny for me. The blacklist functionality for Tekkit. After everything that's been done, you really think a login message is going to get people to stop using Tekkit? Bukkit is dead, Tekkit is stuck on 1.2.5 and Forge-like support is finally getting to where it should have been all along. If you don't want IC2 being used, then do it in a way that might have effect. Crash the client with a message similar to the Multiplayer Mod Incompatibility check... but that's beside the point.
The fact that capes outnumber blacklists by better than 2:1 shows what the important part of that list was.
This is a bad-practice for an aspiring programmer to be getting into. The blacklist and the cape functionalities both could have been added without online checks. Blacklist, Playerlist and Cape Art all could have been hardcoded at the loss of instant updates.