Make sure violence_hblood is not set to 0. You can check it in console, but you might have put it in a config file somewhere. If you don't see bullet holes or sprays either, make sure r_decals is set to 300 or some other high number.
Here's a little trick I use: add the following four lines to your userconfig.cfg:
violence_hblood 0
violence_hgibs 0
violence_ablood 1
violence_agibs 1
This will make enemies bleed yellow blood (like from the aliens from Half-Life) that is easier to see through and harder to mistake for enemies.