At least three bands have gone by the Black Tongue moniker:

A band from Hull, England. Founded in January 2013, the band features members of Infant Annihilator and Mister Sister Fister, Namely Alex Teyen (vocals), Eddie Pickard (guitar, vocals), Aaron Kitcher (drums), James Harrison (guitar), Lloyd Newton (bass guitar). Constantly labelled as one of the most original and groundbreaking bands in the underground scene, these up-and-comers have amassed numerous endorsement deals, and various label interests almost since day one. Their debut EP “Falsifier” has been rated 11/10 and 9.5/10 by independent reviewers and is receiving a ton of praise from both deathcore and traditional metal fans alike.
Musically combining aspects of deathcore with sludge/doom metal. This style is often referred to as "downtempo deathcore".

Black tongue is also rock band from Copenhagen, Denmark. The line-up consists of: Cristian Olesen (guitars, vocals), Rune Nielsen (bass guitar), Snakebite (drums), and Jonas Skovsbøl (guitars). The band recorded their first self named EP in May 2009. The EP contained 4 tracks and the band is currently unsigned.
http://www.myspace.com/blacktonguedk/

An American act is also named Black tongue