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I'm sure you realize that we can't actually diagnose here, but sometimes the information we provide is enough to give a little guidance. I assume that rattlesnakes, and spiders are not an issue?
In California "foxtails" can present this way, they are grass awns that are nasty and get embedded under the skin.
An abcess can sometimed have a dark center above it in the skin, that is skin that will actually die. I am not sure I am helping.