With any luck, if you can get Jorah's minerals balanced, his colour will improve; but, it might take up to 3 years for it to be really black, all the time.  It all depends on whether he is iron overloaded. 

I will have to find Kathleen's pictures of Hank. He was a lovely bronze colour when he was purchased, but it turned out he is actually a very dark bay or dark brown. Since mineral balancing, he is almost black, which is the colour he should be. Others have experienced similar colour changes with their dark brown or black horses finally becoming that colour once the minerals were balance.

Good luck with the new pelleted minerals - we will all keep our fingers crossed for you, as we all know how frustrating this can be.