A type of beverage made from a pale ale and a lighter (nitrogenated) dark beer such as stout or porter, typically Bass and Guinness
In Banging Camp, this is undoubtably the meaning explicitly meant by the character in the song. But due to the fact that the songwriter felt it necessary to clarify the meaning, "they mean the kind from the cans", it could implicitly be a reference to the Black and Tan regiments used in Ireland to suppress revolution or uprisings amongst the Republicans. This varient would fit thematically with the earlier lines about the "great white sharks", which act as a brutal, repressing force. There's also a strong connection in America between Catholics and people of Irish descent.
It could also just mean that they drink pre-canned black & tans (which are, for all intents and purposes, just two beers mixed together without seperate layers) rather than mixing their own.