In basic geology terms, volcanic rocks that form from cooling lava can be very hard and sharp. They can be ground down over time to form harmless sand, so the fragments are small enough that they’re safe to walk on with bare feet without hurting you.  The only inconvenience you might experience is that black sand absorbs more heat than white sand, which typically reflects it. So chances are slightly higher that you might burn your soles a little if you don’t bring shoes, but you certainly won’t get cut.