At my school it is a giant purple page with the website's name and blocked reason.
My school does a simple reading of META tags and looks for keywords to block.
Returntoblockland.com:
blockland, rtb, tbm, aio, returntoblockland, return to blockland, add-ons, mods, weapons, guns, vehicles, kaje, trader, space guy, spaceguy, ephialtes, ephilates, ephiliates, addons, download, blockbot, mod manager, rotondo, badspot
The keywords must have at one point contained "chat" and my school has blocked it even though it no longer contains "chat". The blocking sucks. For example, returntoblockland.com is blocked while forum.returntoblockland.com is not.