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A necrotic spider bite like that of the recluse family (loxoceles) or hobo family (tegenaria) can leave this type of mark which may grow in size but other insects can leave black marks in the central puncture/ wound site like wasp or hornet stings, ant bites, etc. Have it seen for evaluation.
This is very curious, my symptoms present the same as yours. In 2010-2011 I visited Costa Rica with the family, and got an insect bite on the inside of my left foot. The bite took easily 3 weeks to resolve, incredibly itchy. Every several months the bite comes back and itches a lot, same place every time.