What Causes Black Circle On Bottom Of Heel And Swelling On Ankles?

My son has a small black circle on the bottom of his heal that has been there about a week. I thought it was a blood blister because he plays outside without shoes all the time. Now his ankle is red and swollen and he cannot walk on that foot. What could this be? We are naturalists and I have found information online about using black tea bags, bamboo foot pads, or colloidal silver on it for a spider bite but I am unsure which of these would be most beneficial to start with. I am also not sure it is a spider bite from the pictures I have found online, although where we live is overrun with black widows, brown recluse, and other spiders.