I'm replying to myself, in the expectation that anyone following this thread may find my solution useful. It sprang from reading the Comments of purchasers of HP printers, who aired their (dis)satisfaction. This one mentioned the "drawback" of having to have all four cartridges containing sufficient ink, before even a new, full black cartridge will print. (There are potential advantages in this, but they are mostly for the ink salesman.)
I had forgotten how to check the ink levels: the computer screen refers to a"Control Panel", which I assumed meant the WIndows (7) Control Panel, as I don't normally watch the little illuminated screen on the printer. Once I discovered this, and pressed the buttton for the [spanner] icon, I was able to see which cartridges were deemed inadequately full. Once I had replaced these (colour) cartridges, black ink printing performed as required.
Idiot's lesson: Check the ink levels--you may not have noticed a warning!
One does not forget how much frustration was experienced in searching for the HELP that one expects to find.