I have constant pain on my lower right side of my back, just below my bottom rib. Doctor done loads of blood tests, all of which are negative. My kidney function was also good. Would a severe kidney problem show up on bloods?

Not necessarily, I had a blocked kidney causing me pain and never showed up on blood work. They only caught it on a ultrasound followed up by an mir. Mine was on the left side though from a upj blockage.