It depends on the type of "blight". Early blight fungus can survive on the seed coat unless you disinfect them. Late blight spores can only survive in living plant tissue but it doesn't hurt to disinfect them prior to planting as well. Disinfecting seeds is easy - simply a brief soak using a diluted bleach or hydrogen peroxide mix.

Given all the fungal and bacterial problems of this past year (due to the weird weather in much of the country) seed disinfecting prior to planting should almost be mandatory next year...just in case. ;)

Dave