"3 main trunks" is probably just one plant that has put out 3 canes. My Chester puts out about 6 or so canes each year from the centre of the plant.

My Chester blackberry here in zone 3 grows about 15' from one end to the other each summer. People in warmer climates report them growing 25' across. You are going to need one amazingly big pot for that.

You can grow them in combination with a grape vine in the garden... that's actually how I grow one of mine. If you don't prune, the grapes will always win. I planted the blackberry a foot or so in front of the grapes, then let the blackberry have the front (south-facing) part of the trellis, and prune the grapes back whenever they threaten to smother the blackberry. That planting is not nearly as productive as another Chester that has it's own trellis, but it still works.

Concords will soldier on in neglect until they become a mess of spider infested moldy weeds.

Have you had a go at them with some mercilessly viscous pruning by the book? Your statement "the foliage... does a lot better than the fruit" says you most likely haven't. it could also imply that you've fertilized them. Hope you haven't.

Merciless! Vicious! By the book!

Just a guess from what you wrote.

The grapes are totally neglected. I don't even think we've done any harvesting in at least 5 years. The last time I remember having home made grape juice was right around when I got married 10 years ago. Other than that, my dad doesn't prune, fertilize, propagate, etc. the grapes. He just tills around them and does the rest of the garden (usually corn and pumpkins are near the grapes)

There is a little fence on the west side of the yard with nothing else there. I'll ask my neighbor if she minds blackberries there :)
There's also fences by the street, but I don't want random people eating MY berries. I'm greedy! ;)

The picture is last night. You can see my dad's garden in the background, with the grapes along one end.

Here is a link that might be useful: {{gwi:92050}}