Green’s Dictionary of Slang

Black Maria n.

also black mariah, Maria, mariar, sable Maria
[SE black, the colour of the van, but the ety. of Maria is unknown; suggestions include an abbr. of married, two or more prisoners chained together; a play on the -ria of Queen Victoria’s name (which fails in the face of its origins in the US, although V.R. was inscribed on the British vans) and Brewer’s suggestion in Dict. of Phrase and Fable (1894) of a derivation f. one Maria Lee, a Black madam of Boston, Mass. So large and fearsome was Ms Lee that she was regularly called upon by the local police to help them first arrest and then take criminals to prison. According to F&H, themselves citing ‘a writer on slang’, the term was coined c.1838 in Philadelphia, although the OED first use is 1847 (usefully for Brewer from a Boston newspaper) and DSUE (1984) notes Joseph Neal’s story The Prison Van, or The Black Maria (1844)]

1. [mid-19C+] (orig. US) a prison van for conveying prisoners.

2. [mid-19C+] (orig. US) a hearse.

3. [1980s] (orig. US) an ambulance.