I have a portable media blaster I use on various things. I've seen a lot of blasted rocks and block, you'd just have to be careful as hell with doing it installed like that, cus the grout is going to break off way easier than the stone.

It's probably not going to be possible to go lighter, but darker would be easy. Concrete stain as the other guy mentioned. If they're manufactured stone , the color is likely mixed into the material, so you probably can't remove the outside layer to reveal a more base color. You could try chipping away the surface on a more out of the way stone, see if it's a different color underneath. If it is, then you can abrade the color off with a sander, chisel, or sand blaster. If it's solid color all the way thru, your only chance is to dye it darker

When cutting the 1" purple foam I've been using the big snap blades, where the razor is scored so you can break off sections for a new edge. Extended all the way out so you can get a nice shallow angle on the pull