Black spots under the nail cap of the thumb on the foot may be caused by subnail hemorrhage, nevus of the nail and other diseases. 1. Subnail hemorrhage: if the nail cap of the thumb on the foot is squeezed, hit or smashed by the outside world, it may lead to local redness, swelling, bruising and other symptoms, which may cause black spots under the nail cap of the thumb. 2. Nail nevus: Nail nevus is a kind of nevus, the cause of which is still unclear, and may be related to trauma, infection, drugs and other factors. Freckle-like nevus or pigmented nevus will appear at the root of nails of patients, and with the growth of nails, it may form black lines and black spots under the nails. Regular observation is needed to rule out the possibility of nail melanoma. For patients with black spots under the thumbnail cap on the foot, it is recommended to go to a regular hospital for examination to clarify the cause.