Bloop is a made-up word normally used by a cast or staff member to poke fun at Eagle Beagle when EB says something random, inane or irrelevant to the conversation going on during the show. When caught, the staff member will usually shout out, 'BLOOP', and the viewers will then follow suit in the IRC chat room.
It is supposed to have a certain sound, like “Pop Up Video” as that video tv show would pop up random information in bubbles, with a “Bloop” sound, which interrupted the music.
Bloop is also known to have become a verb. Occasionally, when somebody other - i.e. not Eagle Beagle - says something random, a cast member might ask them “Did you just bloop?”