My dad had a tomato garden in the back a couple of years ago. The
garden is still there but he hasn't grown anything in it for a couple
of years. Imagine my surprise when I saw what looked like tomato
leaves. I walked over to see if he'd planted it and I saw a plant with
little black berries. I thought I knew what it was and went back
inside and looked up images, and sure enough, it was nightshade. I
think it's a variety called Black Nightshade but not 100% on the
variety.

We don't really care what's growing back there now as we're not going
to eat it anyway, but I was curious to know if it would be safe to
garden in that soil later. Would the root or leaves or other
decomposed parts of the nightshade plant taint the soil or poison the
soil around the plant?

I was thinking of maybe growing something back there myself but I
don't want to grow it in poisoned soil.

I don't think it would do anything like that, as the poison would be
in the plant itself and not in the soil surrounding in my opinion, but
I wanted to be safe about it and see what other gardeners thought.