One of my blackberries shows a strange malformation of some (but not all) of the flowering canes:
Flower color is purple instead of white and what would usually be the forming berries is strangely elongated and spiked:
On those canes, the new growth is also somewhat stunted.
The plant has been on the site for a few years and is an offshoot of a plant I had for seven or eight years (on the other side of the house), so it’s nothing that came from a nursery.
Other canes on the same plant show perfectly normal flowers and habit - compare the normal ones in the foreground and the stubby ones in the background:
I have a horrible suspicion: Could it be rubus stunt? But before I start digging up a salvageable plant, I’ll wait for this community’s input.