How serious a disease is white blister? Can I still get a crop of brussels out of infected plants (with leaves removed)?
Pretty serious I think. I'd get rid of the affected leaves for certain, don't put them on the compost heap - its a fungus and will live on debris and come back again. If once the leaves are removed, the plants appear healthy and grow new unaffected leaves, then just carry on, but if more appear I'd bin the plants. There's nothing you can do that I know of to treat them with.
You aren't too late to start again with brussels that are resistant. There's one called Bridge which you can get from D.T.Brown or Fothergills.
Boo hoo, have been removing affected leaves but have now discovered that some of the sprouts themselves had got it - whipped the plants out, harvested the usuable sprouts and bagged & binned the rest. Caulis not affected. Not all sprouts are affected either.
At least you got some sprouts - every last one of mine has been eaten by slugs. A sproutless Christmas this year I'm afraid.