They may grow wild, but I would definitely recommend working with cultivated varieties. The wild varieties of blackberry have hugely variable fruit; flavour, sweetness, size, quantity of pith, toughness etc... For a market garden you want a reliable product you can
sell, so cultivated strains are hugely beneficial.
As for a guild, my
experience is that blackberries grow best natively in either exposed soils or in shaded woodland. However they don't tend to fruit prolifically in shade. That gives you an indication of the conditions they like - their
roots systems seem to thrive in soil with high
carbon content (forest floor). They don't do as well with
root zone competition from grasses and the like.
I would consider a plan with plentiful use of comfry (shades the soil, reducing root zone competition with grasses) and mulching with woodchips. Well rotted chips are best, to avoid the nitrogen leeching that can happen. I would plant comfry in the path area, where the stems will flop sideways, then mow/slash it down around picking time.
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