Hi. 3 Months Ago My Husband Sustained A Very Bad

Hi. 3 months ago my husband sustained a very bad bruise on his upper arm that has never really healed. It changed from a deep purple/black bruise to a pink rash with tiny blisters on the surface of the skin. A light discolouration has remained since then and as of a couple of days ago the rash seems to be returning. What could this be? Thanks in advance. Lis