Black itchy arms may be associated with solar dermatitis, fungal infections, eczema and other conditions. 1. Solar dermatitis: If the arm area is exposed to strong sunlight, it will lead to the redness or blackness of the skin in the area, the surface is accompanied by obvious pain, itching and burning sensation, and in severe cases, there will be peeling phenomenon. 2. Fungal infection: If the arm is infected by fungus, fungal infection for too long, the affected area of the skin will become black and purple, thickening of the skin, the surface of the skin has a rash, which can be accompanied by itching symptoms. 3. Eczema: arm with eczema, the early stage will appear the base of the reddish rash or herpes mound, when the temperature rises, itching aggravated. Repeatedly scratching the affected area, the skin in the area will have ooze and scabs, and over time, the skin will appear hyperpigmentation. There are many other causes of itchy black patch on the arm, which should be taken seriously, and you should seek medical attention to identify the cause and follow medical advice for further management or treatment.