I have painted eggcrate and PVC pipe black using the flat black Krylon Fusion. After it cures it is 100% safe for the tank and you will not have any issues.
With that said, the paint does wear off over time which will leave the paint job spotty. As you clean off the eggcrate, you will accidentally chip it. Frag plugs being wedged in will scratch and chip it... It will look good for awhile but eventually you'll be painting it again. For this reason I have moved to black eggcrate, however I still use the black painted PVC stuff.