Hello and welcome. Thank you for your query. I understand that your child was diagnosed to have impetigo. From the information provided, it seems most likely that the child may be developing ecthyma- a bacterial infection that usually causes skin and soft tissues to have a black discoloration. Common signs of worsening include increase in the size of the sore, pain, swelling, fever, not eating well and appearing sick. When seeing patients with similar presentation, I usually advise oral antibiotic treatment, including a fluoroquinolone , and an anti MRSA antibiotic, for ecthyma especially if not responding to a topical medicine. I would suggest seeing your doctor for prescribing antibiotics and evaluation especially, if your child has any of these above mentioned signs of worsening. Another rare cause of turning black would be a reduced blood supply and gangrene formation which would be usually very painful and is an emergency and needs to be evaluated. I hope this answers your question. Please do write to us if you have any further questions and I would be happy to help. Best wishes