The reason the blood in the blister is turning black is that it is congealing. The blood is clotting and solidifying. The reason th blister is expanding isbecause the wound is reopening when u train and bleeeding more. Easy fix, drain the blister with a neddle prick or whatever.

The issue are the blisters themselves....why are the occuring? Blisters occur because of friction and pressure. In other words they only appear if there is something wrong.....Boots too small, sharp edges in the boots etc. Address the blisters first, then address the cause so they wont reoccur