Jamieson, Fausset and Brown on Job 2:7

7. sore boils--malignant boils; rather, as it is singular in the Hebrew, a "burning sore." Job was covered with one universal inflammation. The use of the potsherd [Job 2:8] agrees with this view. It was that form of leprosy called black (to distinguish it from the white), or elephantiasis, because the feet swell like those of the elephant. The Arabic judham (De 28:35), where "sore botch" is rather the black burning boil (Is 1:6).