It's bad Latin, though.
Arcanus in Latin is an adjective that just means "mysterious, hidden, secret, private". It's not a noun, and has nothing to do with magic or rites, which is the English usage (where it means "specialized knowledge", which is how it's being used here). Moreover, even if you were to use arcana as a nominalized adjective (which is possible), the inflections are wrong: it should be nigra arcana. Though again, that would just mean "the black secret things", and not "black magics".