Check out this post: Darktable, Negadoctor, and Black and White Flatbed Scans: A workflow demonstration and watch his video where he has to manipulate the colors (I ask him about converting it to monochrome instead in that post).
He uses a flatbed scanner, and saves the images as color TIFFs, I believe. Which, I believe, would have the same results when brought into darktable as you shooting a b&w negative with a color camera and bringing it into darktable: it’s technically a color image you are dealing with, so yes, I believe, it needs to become monochrome at some point in the darktable workflow.