What could blisters on crack of bum be from? it is as though it's clustered the bumps are hard and itch.

Get checked: I will often see herpes present this way and occurs repeatedly as a cluster of small blisters that itch, burn, or tingle and then become crusted and then red scar and happens a number of times a year (or even cyclical during menstrual cycle). If it is hsv, they may be contagious and require treatment. There are also many other possibilities so please see a dermatologist.