I had 2 tomato plants in pots in the garden and both got late blight. I usually tip the used soil from pots on the garden or plot is it still ok for this?
Yes. Blight only survives on foliage. As long as you have cut the tops off and disposed of it.
More precisely, blight survives in living plant material. To be safe, I'd knock the soil of the roots first, not just cut off the tops.
The usual culprits are potatoes that evade harvest, then come up next year.