What would the Medieval Latin translation be for the title "Book of the Black Sacrament"? (Note: black here means dark or evil, not the actual colour.)
Thanks.
If you're looking for a church-flavored phrase, I'd go with Liber de Sacramento Obscuro.
Obscurus can mean "dark (as in black)," but also "hidden," "secret," "disguised," or even "ignoble." It doesn't mean "evil" per se, but it approximates some of the meanings of "dark" and looks like the English word obscure to boot, which would help your readers.
Other just-as-good words for black here include nigro and atro in their inflected form.