A:Answer From the Brother support site concerning the HLL3270CDW model:
If your machine's cyan, magenta, or yellow toners run out, you can choose Mono in the printer driver to allow you to keep printing. You'll be able to keep printing until the black toner runs out. When the black toner runs out, you won't be able to print until you replace it.
So according to Brother, yes, you can print in black without the color toner being available.