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Hi there,
The exact cause of this black mark on sole of foot can be determined only after a complete clinical examination. This spot can be purpura due to a blood-clotting disorder. The condition makes you more prone to bleeding or bruises .Other possibility is that this could be a new mole. If it was a bruise, there would have been a gradual change of color over two weeks from black to brown to blue to green, etc. so a bruise seems unlikely. It might also be a condition known as seborrheic keratoses.These start as small red bumps which gradually thicken and become warty. They are white or black in colour.You should consult your doctor and discuss about these conditions: moles, bruise, seborrheic keratoses and melanoma. Do write to me again.
Best luck and regards!