08-31-2017 12:47 PM
Welcome to HP Forums, this is a great place to get support, find answers and tips.
Thank you for posting your query. I'll be more than glad to help you.
I understand that you are facing issues with your HP Officejet Pro L7780 All-in-One Printer wherein it is not printing in black after changing the black ink cartridge.
This issue could occur if the printhead is expired or the nozzles may be clogged.
Let's go through a few steps that may help to fix this issue.
And here are a few steps in the link: Fixing Print Quality Problems
The print quality diagnostic report will tell about the printhead health. Accordingly, you need to replace it.
For printhead replacement, you may need to contact HP phone support from this link: www.hp.com/contacthp
You may select the country and type the product# of the unit and then follow the on-screen instructions to contact HP phone support.
You have a good day ahead.