I have a Canon i865 which is now about 4 years old and has had a reasonable amount of use.
It's worked well, and I've been very happy with it.

The large black cartridge ran out of ink, so I replaced it as I've done quiet a few times.

Now I find the printer won't print BLACK.

I ran nozzle clean, and deep clean.
Still won't print BLACK.

Ran head aligment, which failed with an "Error Number 2500". Which I can't find an explanation for.

I tried some black TEXT, and got a blank page. I changed the colour of the text to red, green, yellow and then blue.
All printed OK.
Tried BLACK again and got a blank page.

Just in case; I replaced the black ink cartridge. Still won't print BLACK.

Now, the i865 has two BLACK ink cartridges, so I checked the other one and it seems fine. Plenty of ink.
(Not sure which gets used for text).

Everything is working, it just won't print BLACK.

Any ideas? Is this a sign that the print head is dying or that prehaps it's blocked?