Thank you to everyone for your comments above or via private message. I'm fairly confident that it isn't contamination of the developer by blix (but of course it might be...). I also don't use a safelight. I constructed a kind of cradle that sits on top of the processor that helps me get the paper in the right slots in the dark, so I know I'm not sticking it in the blix and then in the developer!
I'm going to insulate the sides of the Nova processor to make sure that the dev and blix slots are the same temperature and I will try replenishing at 10ml for each 10x8 sheet that I've already done. I will do without a stop bath until I can find an acetic acid one. (Is it possible for the stop bath to poison the blix?).
If that doesn't work, I'll chuck out the developer (well, give it to the haz-chem man) and use some more from the 5L that I mixed initially, as an experiment. Then I'll do the same with the blix. Failing all of that, I'll make up some fresh and start again.
I'll report back.