My HP Officejet All-in-one 6830 has worked well for over 6 years of moderate use. Recently it stopped printing black, even though a new cartridge was installed and was detected as full. All other colors worked fine.

A quick search showed the issue to be, if not common, then at least well-known. Suggested solutions varied from simply running a few cleaning cycles via the printer maintenance menu all the way to a hardware fault with the printer.

What did not work in my case:

1. Trying several different ink cartridges (to rule out cartridge defect)
2. Running multiple cleaning / alignment cycles via the printer OSD menu
3. Cleaning the printhead with alcohol via the syringe method as recommended in various instruction videos, e.g., here, here.

What worked in my case:
Remove the printhead, rinse it thoroughly under running water for a couple of minutes until no ink comes out, wipe out any residue on the contacts, and let it dry thoroughly (for a few days) before reinstalling.

After I did it, it was suggested that it would have been better to use distilled water and simply soak the printhead in a bowl of it, since tap water can cause its own problems. Since I do not have any distilled water at the moment, I did another round of cleaning with small amounts of alcohol, just in case. I suspect that I was dealing with some mechanical blockage of the ink nozzle, and in this situation running water can help more than just soaking.

After putting everything back together, all colors print flawlessly. No idea how long it will last, but hopefully long enough. I'd hate to toss the printer into the bin with a nearly full set of ink cartridges. 😏

In this particular model, the printhead is a b!tch to remove and reinstall - you have to first remove the cover first and deal with a couple of nasty springs, but there are good instructional videos online, e.g., this. A good idea is to first lift the top flap, so that the printer moves that printhead out, then yank the power cable out. This way you don't have to manually release the printhead slider. Some of the Torx screws are embedded deep, so you need a screwdriver with a long tip. And there are several annoying flat cables to deal with, but nothing too complicated.