Brother MFC-685 will not print in black, not even on test page. But will print when i scan print

You almost certainly have a defective printhead. When you make a color copy or print, the machine makes black by mixing your color inks.
If the machine is under warranty take it to an authorized service center (listed on Brother's web site) for free repair. If it's just barely past warranty, call Brother's Customer Support and plead for mercy. They often bend their rules to help a just-past-warranty customer.
If they won't cover it, steel yourself for bad news. The parts are expensive, and the required updates and adjustments are a time-consuming pain. You'll probably be told it costs more than a new machine. Good luck.