There are a variety of reasons why your Brother Printer not printing black(https://www.customerservicephonenumber.co/blog/brother-printer-not-printing-black/). Here are some of the most important reasons, as well as a few things to keep in mind before getting too worked up over the issue. Keep in mind that problems nearly often start when a fresh cartridge is installed.

Completely remove the breather tape before commencing operations

Keeping refilling the soon to be runout cartridge.

Weak or defective Pump for Purge

Using generic ink cartridges of poor quality.