Potato blight and soil

Can I use the compost in my potato bag for growing anything else - e.g. bulbs etc. The potatoes were fine, but the leaves had blight.

It depends what sort of blight it was. Late Blight is the typical potato disease. Its spores don't live on in soil, only in infected produce and plant matter. Potatoes can  suffer other fungal diseases, too, and the spores can settle on top of the soil and live on. To be on the safe side, why not remove and replace the top couple of inches of soil?