Welcome to HCM
It is very good that you donated blood,but black and blue colored pathch which you developed because of some blood leaked from punctured vein in tissue below skin,noramally a firm pressure applied for sufficient time can prevent any leakage of blood, now it will go away with time you can apply a heparin
based gel Thrombophob locally to hasten the recovery it should not be a problem apart from it looks bad cosmotically for few days.
Please continue to donate blood volentarily and next time just take care.
Good luck to you.