Occurs only in connection with the sixth plague of Egypt (Exodus 9:9,10). In Deuteronomy 28:27,35, it is called "the botch of Egypt." It seems to have been the fearful disease of black leprosy, a kind of elephantiasis, producing burning ulcers.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Blains'". "Easton's Bible Dictionary".