Blight is an air-borne fungus which needs the leaves of its victims to be wet in order to infect the plant. It doesn't come upwards from the soil unless you have old infected material in it.

Therefore, there is no difference in the likelihood of infection between sack-grown or ground-grown potatoes. If blight hits, the only plants which will avoid it would be those grown undercover.

Remember that PSB is a very big plant - about 1.5 m height and 60-70cm spread. Don't make it compete for water and nutrients.