That message is caused due to taint. Taint, in and of itself, isn't always a bad thing - it's quite expected. But when you get the "An action was blocked" message, that means that an addon is doing something that taints in such a way as to hinder the UI.
In your chat frame, type
/console taintLog 1 then exit and restart your game. Log back in and do whatever you did to get the error message before. Usually, this means that you can go outside of a major town and let low lvl mobs hit you for a few minutes while you do stuff with your UI that you were doing before. When you get the message again, type
/console taintLog 0 and exit the game. Your taint log file will be in the Logs folder in your WoW directory. It will tell you what addon caused it and how.
edit: for more info, see here:
http://forums.worldofwarcraft.com/th...76470259&sid=1