The contact are of copper alloy like brass I suppose? Being black suggests they use the wrong material for repeated unplugging. A gold layer would be better, or some hard precious metal.
If the contacts are well accessible, I clearly prefer mechanical cleaning. (1) Eraser if possible, it's the most caring method (2) The scratching back of a sponge, works often (3) Fine sand paper. I haven't tried scouring powder up to now.
If not, you might try iron perchloride FeCl3. It tends to work faster than HCl. Rinse well any etchant, as it builds leak paths for electricity, and some electronic circuits are extremely sensitive to them.
I didn't try rosin and heat in this use, but this serves to deoxidize copper alloys when soldering.
Aluminium foil in hot (100°C) sodium bicarbonate or hot soda deoxidizes silver nicely. I didn't try on copper alloys.