I have three thornless blackberry bushes and there are no blossoms at all on any one of them. The foliage looks good, however and I have followed all of the suggestions for blackberry care, such as pruning, trellising and fertilizer. Jaykay
No Blossoms on My Blackbereries
Certified GKH Gardening Expert
If you are pruning properly, the lack of blossoms could be an issue with fertilizer or with the age of the plants. It's possible that your soil already has sufficient nitrogen, and that added nitrogen is suppressing flowering. You could try adding a phosphorus fertilizer that doesn't include nitrogen.
Here are some general reasons a plant may not flower: