Black pox is a fungus disease caused by Helminthosporium papulosum. It is considered to be of minor economic importance. When it occurs, the disease can affect apple bark, fruit, and foliage. It is more common from southern Virginia southward than in the northern mid-Atlantic region. The same fungus causes blister canker on pear. For more information, read this article on Black Pox of Apple.
Answer provided by Alan R. Biggs, West Virginia University.