The story
From black + mouth. For the origin of the pejorative "Presbyterian", several theories exist: * A specialisation of a dialect sense "liar, slanderer, foul-mouthed person" applied to political radicals. * Ulster Scots refusing to take the "Black Oath" imposed in 1639; * Covenanters eating blackberries or blaeberries while hiding in the wilds during the The Killing Time.
Etymology from Wiktionary, CC BY-SA 4.0.