Black Rings at the Top of Blackberry Bushes
The top of the bush will grow beautifully, then eventually will get black rings around it and the top will droop. The bush below will look fine.
Certified GKH Gardening Expert
It might be spur blight or even cane blight, but from what you say, it sounds more like spur blight. This article has more information on spur blight on blackberries: