If plot had bad blight this year, can the soil be cleaned of blight and how soon can I plant potatoes again?
Answer was: Spores wash into soil. Wait 3 years before replanting. Always bin or burn diseased foliage.
Actually, spores quickly die off in soil unless they infect a new potato tuber. You could plant potatoes in the same plot the following year and you would not get blight. However, if any tubers left in soil at harvest get infected then the blight can (low probability) survive and grow into the volunteer plant the following year. Replanting will encourage any potato cyst nematodes in the soil and certain other diseases will be promoted so it is always better to rotate.
No need to bin/burn blighted foliage for same reason. The pathogen needs a living host to survive and will not survive in the compost heap or even in foliage left to rot on the soil surface.
There are very very rare exceptions to the above that need not worry the grower.