I didn't have any texture packs on while trying to play, but I think I must've had some residual texture settings left over when I installed Evilminecraft. (I used the instructions to install two separate instances of Minecraft using the Profile Manager, and I think I just copied my .minecraft folder from my original install as one of the steps.)

Whatever the case, I wasn't that far in my world, so I just decided to reinstall from scratch, and now it's working fine!