This page can’t explain what is happening in your particular wound. What it can tell you is what to do.
Black edges around an injury are a sign of dead tissue. Contact your provider right away if you have black edges around the injury.
Black tissue in the wound is also one of the findings that should prompt you to contact a doctor or nurse quickly, along with a foot or toes that become very cold, pale, blue, or black. These signs may mean a serious problem with blood flow or infection and need urgent medical assessment.
One situation needs special mention. Call your health care provider right away if you have a foot infection that becomes black and smelly. These are signs you might have gangrene. Sometimes a bad infection never heals, and the infection might lead to gangrene.
With diabetic foot ulcers, wounds that are pale, purple, or black are less likely to heal. Contact your provider if your foot ulcer is very white, blue, or black.
None of this tells you which of these applies to your wound. What is clear is the timeframe: this is a finding to have looked at promptly, not watched at home.