I have a canon multifunction MX700. Suddenly, my black ink stopped printing. Printer prints in other colors. I have turned the power to both printer and computer on and off, have checked the USB cable, clearned the print cartridges and replaced the black cartridge. None of these have resolved the problem.'Any ideas on how to fix?

I have completed the steps you recommended and still have not resolved the problem. The nozzle appears fine.
What is the next step? I have completed the steps you recommended and still have not resolved the problem. The nozzle appears fine. What is the next step?

To check the black nozzle you should print nozzle check pattern (see the sample above). In the black nozzle pattern, all lines and columns should be all present. If some lines or columns are absent means the nozzle partially missing but if nothing is printed at black nozzle black section means the nozzle totally missing.
If the nozzle partially missing, we can localize and pin point that the print head is faulty. But for the nozzle totally missing may be caused by the following: the print head is faulty, logic board (main board of the printer) is faulty, or purge unit does not work properly.
To define the problem accurately, you need compare the print head. Find the good print head and install on your machine. Print some document (about i.e: 5 pages of text ) and the print nozzle check pattern. If the nozzle pattern is good then you know that your own print head is faulty and you should replace it with new one. But...if the problem still persist, you still have 2 possibilities: logic board is faulty or purge unit does not work properly.
To check whether purge unit works properly or not, you should drain the print head manually (i.e: by using blower, suction kit,etc). After that install the print head and perform cleaning. Then print nozzle check pattern. If the nozzle check pattern is good means the purge unit works normally. Or else, purge unit is faulty.
After all, please also consider using print head cleaning liquid that you can buy at computer store. When you are sure the print head is faulty you may replace the print head with new one. To check the black nozzle you should print nozzle check pattern (see the sample above). In the black nozzle pattern, all lines and columns should be all present. If some lines or columns are absent means the nozzle partially missing but if nothing is printed at black nozzle black section means the nozzle totally missing. If the nozzle partially missing, we can localize and pin point that the print head is faulty. But for the nozzle totally missing may be caused by the following: the print head is faulty, logic board (main board of the printer) is faulty, or purge unit does not work properly. To define the problem accurately, you need compare the print head. Find the good print head and install on your machine. Print some document (about i.e: 5 pages of text ) and the print nozzle check pattern. If the nozzle pattern is good then you know that your own print head is faulty and you should replace it with new one. But...if the problem still persist, you still have 2 possibilities: logic board is faulty or purge unit does not work properly. To check whether purge unit works properly or not, you should drain the print head manually (i.e: by using blower, suction kit,etc). After that install the print head and perform cleaning. Then print nozzle check pattern. If the nozzle check pattern is good means the purge unit works normally. Or else, purge unit is faulty. After all, please also consider using print head cleaning liquid that you can buy at computer store. When you are sure the print head is faulty you may replace the print head with new one.