In my haste to fix my problem, I forgot to take a photo of the toner cartridge that shows the problem but below is a pic showing the cartridge after I'd cleaned it and done a few test prints - you can just see the toner is starting to build up at the edges of the drum again, assuming that the blue thing is indeed the drum.

I cleaned the toner off using a soft tissue and did a few test prints which all came out OK. When I checked the toner cartridge I saw that the drum (I think - I'm not too familiar with the different parts of a cartridge) had a solid black band of toner across it. Thanks for this info - I did some further investigation and found out the problem. Is anyone able to check on a working LaserJet 200 to see what the roller looks like? Before I purchase another cartridge, I wanted to check whether the roller in the back of the printer could be a problem - the roller in my printer is totally black and seems to be covered in toner - see below. I purchased the current black cartridge from Amazon in Nov-2016 and it hasn't been used much since then. I've seen a couple of other posts on this forum, one of which looks to have been related to the toner cartridge.

Downloaded and run the HP Print and Scan Doctor which doesn't find any problems - all it says is that the supply levels look ok and that the printer is using genuine HP supplies.

Unplugged the printer from the electrical socket and left it unplugged for over an hour before plugging it back in.

My LaserJet 200 M251 has recently started to print pages that are totally black - see attached example of printing the cleaning page using the touchscreen on the printer itself.

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