potato blight

If you are sure it is Blight, then the first step is to remove all the foliage, the spores drop from the leaves when it rains and infect the tubers. If you remove the leaves then you are removing the danger to a certain extent. As long as the tubers are not affected (and believe me you will know when you dig them up!) then they are perfectly usable. They probably will not store though, over the winter. Removing the leaves and stems early will also reduce you crop, but small is better than nothing.