We got a research inquiry today asking ‘what was ‘black tongue’ which threw me for a moment as I’d never heard of it. Fortunately my predecessors recorded information about it.
Black Tongue was a common name for erysipelas – see the two attached documents from the Medical & Surgical History of the War of the Rebellion (3rd Surgical vol.) in the footnote starting on the first page.
http://en.wikipedia.org/wiki/Erysipelas gives an overview and you can see why it would be a dangerous disease before antibiotics.