Darkened nails are not necessarily cancer. There are many reasons for darkening nails, and each case should be analyzed on its own merits. The most common cause of blackened nails is nail trauma. Bruising of the nail bed can cause the nails to look black and blue, which usually goes away on its own, so there is no need to worry too much. Gray nails can also appear cloudy discoloration of the nail, the surface of the nail is uneven, is due to a fungal infection, and has nothing to do with cancer. There is also a condition known as onychomycosis, which is the appearance of black lines on multiple nails, and is usually associated with nail trauma or nail malnutrition. In addition, a nail nevus is a benign tumor that manifests as a black line on a single nail, and in a few cases, transforms into a malignant melanoma. Black nails due to malignant melanoma are mainly characterized by blurred and asymmetric boundaries of the blackened area of the nail and rapid expansion. When black nails appear, it is recommended that patients go to the hospital for examination to rule out malignant lesions, and then take targeted treatment to improve the phenomenon of black nails.