what do i need to do to soil that blited tomatoes grew in

Do you mean you had tomatoes this year that had blight here, or last year?
As long as the plant debris was cleared away it shouldn't necessarily follow that the potatoes will be affected. I find that most of the problem comes in spores in the rain and wind.
It's interesting to observe this year that young healthy tomato shoot cuttings from plants suffering from blight lower down the plant, still begin to suffer with the disease even when removed far away from the original plants. I guess once the spores are there, you can't get rid of them.
Always rotate your crops of potatoes, tomatoes and other solanaceae so that you aren't growing them in the same soil and location every year.
Interesting to note here, that while almost all my tomato plants have been touched with blight, some Roma bush tomatoes growing at the base of sweet corn plants haven't suffered at all.