Green’s Dictionary of Slang

black mouth n.¹

a slanderer.

1642		Declaration of the Apprentices in Harleian Misc. V (1810) 307: Nor are we of the ribaldry of the city (as some blackmouths have uncharitably belched out against us).
1656		J. Trapp Commentary John ix 28: Every black-mouth cast dirt upon Christ’s disciples [OED].
1937–84		Partridge DSUE (1984) 88/2: from ca. 1640.