What can cause a wound to suddenly look black?

Necrotic : Wounds can become black if the edges have died and are now "necrotic". This mean that this wound has lost it's blood supply entirely. See your wound care doctor as these areas may need surgical debridement.

Not good: This generally implies that there has been pressure and/or tissue has died. See your wound doc immediately.

Escar?: A large wound could have developed a thick black layer easily confused with a scab. This is a thick layer or dead and necrotic tissue. It will slow or stop wound progress. Fortunately a doctor can easily remove that by a procedure we call debridement. Do have a doctor preferably wound specialist look at it.

Natural Process: As injured skin heals, the body creates inflammation to do so which means fresh blood goes to the area and with it, many kinds of healing factors and cells. One type is called melanocytes which carry our skin pigment melanin. A healing occurs, these cells get deposited int he area and give it the darkish coloration you see. Over time, there will be some fading as the injury matures.