What can i do about a black scar left by a pimple?

Pigment? Blood?: Treatment depends on what is causing the black pigment. If it is dried blood, then simple washing with half strength peroxide will eliminate the pigment or scab. If it is due to post-inflammatory hyperpigmentation, mild steroids such as a 1% Hydrocortisone and salicylic acid may help but time may be your greatest friend. Avoid sun exposure to the area.