I don't believe it has anything to do with the black walnut tree. Most of my yard is within 30 feet of my big old black walnut and I have a bunch of stuff growing right under it.
I don't know what happened to your plant, but looking at your picture, I don't see any obvious signs of disease - spots, browning, curling, etc - other than the wilting. Since you were implying that none of the other plants in the bed with it are wilting like that, it inclines me to think that something happened to the roots of that one. Either it got root rot (from all the rain), or some kind of root pest (eg wireworm or root knot nematodes), or maybe you were digging around it and cut some roots?
Incidentally 6 tomato plants in a 4x8 bed is an awful lot. I sometimes put five and I am known for crowding things. Having them over-crowded encourages the start and spread of diseases.