Subbing.
Not a silly question-we all search for that.
Does this have to hold up over time? Looking at my accumulated swatches (15) I think some of them faded slightly over time (it has been years) even though I keep them behind a door and out of the light. Bazzill Licorice Twist that Kristina Werner had rec'd years ago seems to be holding up better which is weird being a cheaper paper. If the light hits directly they all look lighter. The Fabriano black drawing paper Dina mentioned I had in a sealed pack out of the light so not a fair comparison but it is a nice dark black.
BTW-seems to me you might be able to "darken" the paper you have if you drag over it with a black pad. I cant remember the word for that right now of course.
if you just have one thing to do....