What's a Black Maria?
A "black maria" is a slang term for a police wagon. How did it get this name? We don't know. But here are two possibilities.
Brewer's Dictionary of Phrase and Fable cites a belief that it refers to Maria Lee, a black woman who kept a boarding house in Boston. "She was a woman of such great size and strength that the unruly stood in dread of her, and when constables required help, it was a common thing to send for Black Maria, who soon collared the refractory and led them to the lock-up." While Brewer's is a great source of folklore, this origin is by no means certain.
Douglas G. Wilson, cited in the Black Maria entry of World Wide Words, suggests that the term comes from a racehorse of that name.
-The Fact Monster