Itchy clustery rash on hands knees elbows and feet that blister wen scratched?

Psoriasis? Although it is very difficult to diagnose rashes without actually looking at it. But it sounds like early psoriasis by the distribution and description that you have given. Please consider uploading pictures and consulting a dermatologist here on HealthTap.
Need Evaluation. This can be a lot of things, first thing that comes to mind is pemphigus, a skin condition that blisters and if rubbed the skin can slough off. You could also have a skin infection or several other problems. You need a good dermatology evaluation.