Leadbelly originally - from Wikipedia:
The origin and meaning of the lyrics are subject to debate. Some sources claim the song is derived from an 18th century marching cadence about a flint-lock musket with a black painted stock; the "bam-ba-lam" lyric referring to the sound of the gunfire. Soldiers in the field were said to be "hugging Black Betty". In this interpretation, the rifle was superseded by its "child", a rifle with an unpainted walnut stock known as a "Brown Bess".

The earliest meaning of "Black Betty" in the United States (from at least 1827) was a liquor bottle. In January 1736, Benjamin Franklin published The Drinker's Dictionary in the Pennsylvania Gazette offering 228 round-about phrases for being drunk. One of those phrases is "He's kiss'd black Betty."

The term can also be taken as a then contemporary reference for a prostitute, a prison bullwhip, or the "paddywagon".

David Hackett Fischer, in his book Albion's Seed: Four British Folkways in America (Oxford University Press, 1989), states that "Black Betty" was a common term for a bottle of whiskey in the borderlands of northern England/southern Scotland, and later in the backcountry areas of the eastern United States.

In an interview (see The Land Where the Blues Began, 1st Edition, Alan Lomax, Pantheon Books, 1993) conducted by Alan Lomax with a former prisoner of the Texas penal farm named Doc Reese (aka "Big Head"), Reese stated that the term "Black Betty" was used by prisoners to refer to the "Black Maria" — the penitentiary transfer wagon.
( Monty Boyce, My cheese game is strong, Mon 27 Jul 2009, 13:59, Reply)