Does anyone know what might be good companions for black raspberries?
I guess black rasberries would be the same as any other rasberries so I hope this helps. Raspberries like Tansy but be careful as Tansy have invasive roots, so don't plant right next to it just a bit further apart. Cut back the tansy to ground level after flowering and use the leaves in your compost. Tansy is said to be an insect repellent. Raspberries do not like potatoes and blackberries so don't plant them near each other. Hope this helps
If going for visual (ornamental) effect bloomy stems of black raspberry main attribute, plants that look well with these would be the best choices. Maybe something like bronze fennel or conifers with purplish winter color, like Cryptomeria japonica 'Elegans' or various juvenile foliage forms of Chamaecyparis thyoides.
Same companions that suit other species of raspberries planted solely for white stems, such as Rubus biflorus would probably be pretty good candidates, climate permitting. British plant combination books in particular will have ideas.