well, all saying from their experiences , let me tell you mine,
once my 2007 cultus (black) faces almost same problem, a road was repairing , and charcoal liquid hitted my car, almost 70% of it, due to black on black it wasnt visible, but one can feel it like rubbing hand on it, what i simply did, uses kerosine oil (mitti ka tail), and havent face any problem till now, no shine/dull problem...faced this problem 2 years back(almost)