Sudden blackening of the big toe can be caused by trauma, onychomycosis, subnail melanoma and other factors. 1. Trauma: When the big toe is traumatized or impacted, it can lead to damage to the local nail plate and nail bed, and local pain, bruising, swelling and other manifestations, which can lead to local ischemia and necrosis in severe cases, thus resulting in the appearance of blackened big toe. 2. Onychomycosis: When fungal infection occurs in the big toe, onychomycosis may appear, resulting in blackened nails, and the nail plate may be thickened and become uneven, which may be manifested as a sudden blackening of the big toe. 3. Subungual melanoma: It is a kind of malignant tumor that occurs in the subungual tissues, which can be manifested as a sudden blackening of the toenail, and at the same time, there will be toenail ulceration, blood seepage, pain and other symptoms. If your big toe suddenly turns black, you should consult a doctor in time, and actively standardize the treatment under the guidance of specialists, which will help to alleviate the uncomfortable symptoms as soon as possible.