Why would a callus turn black?

Depends: Could be due to dirt trapped in the crevices of the callus, may be due to bleeding under the callus, most ominously, it is not a callus but a melanin producing tumor. If you can not wash off the color, have is examined by a doctor. Wish you good health! - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.