FastBuds Blackberry

2 reasons: genetics, of course. Also phosphorus deficiency. With P deficiency, it's often caused by colder nighttime temperatures which make the plant burn through P like you'll eventually burn through the plant. Add some P and the spread will stop.

I can't really see it in the pictures very well. Let me ask you a couple of thing: is the yellowing homogeneous (is the whole leave yellow)? Or is the yellowing more pronounced near the tip and the base still green colored? The former is nitrogen deficiency, the later is potassium deficiency. Do you notice rust colored (like burned) tips and edged in leaves? That's a more pronounced stage of potassium deficiency. At this stage it would be strange to have a nitrogen deficiency, since it's so late into flowering. My bet is potassium