There are many reasons why mint leaves are black and yellow. Look at your pot of mint should be too much water, coupled with the lack of air circulation caused by. And the soil quality is not very good, it is recommended to buy some nutrient soil, better permeability.

Following for you to find several reasons, hope can help you.

If it is because of insufficient light, you need to put it in a sunny place to receive light. If it is because of too much watering, you need to enhance air circulation and loosen the soil properly to let the water evaporate as soon as possible. If it is because of excessive fertilization, you need to take the plant out of the pot, remove the old soil and prune the roots, and then replant it with new soil. If it is because of disease, you need to cut off the diseased branches and leaves, and spray Bordeaux solution to prevent and control.