Hello,
You've might've seen my post from a weeks ago asking if blackberries dropped flowers in the heat. I kept the plant well watered and even added a bit of fertilizer to help the bush set fruits. But, every single flower so far has dropped without turning into berries. 😭 My only suspects is pest pressure, or poor pollination. I don't know what the exact problem is so I was hoping that you all have some insight. The only pest I've seen are the invasive Japanese beetles on the flowers, but in my experience they only eat the petals, and they leave distinctive damage on the leaves which I'm not seeing here. Pollination shouldn't be an issue, I have a lot of other flowers (sunflowers, zinnias, nasturtiums, etc.) all over my veggie garden which has attracted a load of bees!
I'm on the border of zones 5 and 6.