Is there a script function that detects when the player blocks? I'm trying to have a shield that, whenever the player blocks, it plays the slow time spell. I can't make it a constant effect, so I've been trying a script, but the CK wiki doesn't seem to have anything that works.

Take a look at this thread:

http://www.gamesas.com/topic/1360112-full-dump-of-all-animation-variables/

Might be of use. You may be able to detect when a player is doing a block animation and then run a script. You'd probably have to do an onupdate event to update ever 0.5-1 seconds, then make the animation check.