Blister after unmonitored heat therapy

An adult male patient came yesterday with back pain 2 days after injuring his back carrying a light object at work. He did not complain of any 'shooting pain' (radiating pain) to mentioned that the pain did not subside despite placing a hot water bottle (heat therapy) overnight. He had no pain bending forwards (flexion) but pain getting up from bent position (deflexion).

I examined him and found that he had 4 blisters (fluid filled sac in the skin) on his back (see photo) and spasm of his right erector spinae muscle. He didn't know that some of the pain was due to the blisters!

His muscle strain would recover with a week of physiotherapy and some medication for pain and muscle relaxation. He was not advised any treatment for the blister as it would heal on its own. I advised him to avoid using the hot water bottle directly onto skin or longer than 30 minutes to avoid blisters.