A persistent question this time of year is why sometimes blackberry fruit are not ripening. They go through flower, green fruit and then to red, but once at that stage some (or many, I have even witnessed cases of all, really sad) stay red and simply refuse to ripen.
As many readers of this blog know this is most often redberry mite, but not always. Remember that fruit ripening, in other words turning to black, in blackberry is a gradual process, usually taking 3 to 4 days. If one is wondering whether a fruit is actually infested with redberry mite or just slow in ripening, rather than searching for redberry mite in the fruit itself which is a very, very difficult task since they are super small and in a juicy environment, it can pay to tie a few off a few suspect fruit as you see in the pictures below and return to them a few days later.
As you can see from the photos below, you can get interesting results! One fruit is not ripening, while the other continues to blacken and can be picked.
For more information on redberry mite, please see the UC IPM guidelines here:
https://ipm.ucanr.edu/agriculture/caneberries/redberry-mite/#gsc.tab=0
Two blackberry fruit side by side, manifesting what could possibly uneven ripening caused by redberry mite.
Same two side by side fruit 3 days later, note one has continued to ripen while the other has not. Conclusion for me on this one was we have some redberry mite in this field, but it's not every fruit.