The story
At least notionally because such soil was removed at night by nightmen, and because chamberpots were needed especially at night (when one, in one's nightgown, might be less disposed to leave the house for the outhouse); but the general taboo around human excrement encourages euphemism, and any word in the allusive range of the night–dark–black–bad archetypes might serve as well as another for the purpose; thus, compare blackwater.
Etymology from Wiktionary, CC BY-SA 4.0.