Definition of black tongue in English:

black tongue

noun

US
  • (Originally) any of various diseases or disorders exhibiting a dark discoloration of the tongue, especially blackleg in livestock or (more fully "black tongue fever") typhus, typhoid, or meningococcal meningitis in humans; (in later use also) niacin deficiency, especially in livestock or domestic animals; pellagra.

Origin

Mid 19th century; earliest use found in Thomsonian Recorder.