Why would some berries look like they did not mature and turned brown?
Brown blackberries on the plant can be the result of diseases like anthracnose:
https://extension.psu.edu/plants/tree-fruit/news/2013/blackberry-disorders-that-affect-fruit-quality
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberries-not-fruiting.htm
Also, check this blackberry diagnostic tool to see what matches the symptoms your plants are showing:
https://projects.ncsu.edu/project/berries/diagnostic_tool/index.php
My blackberry bush produced fruit first two years and now is a small tree. (unknown tree)
These links have information on growing Black Berry's.
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberries-not-fruiting.htm
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberry-pruning.htm
I had a blackberry bush last year. It didn't get cut back. I am try to find out if it is dead or just needs to be cut back to promote growth. Please help.
Whether they were pruned in fall or not, you should probably be seeing some kind of life to them by now. These articles may help.
https://www.gardeningknowhow.com/garden-how-to/info/when-do-plants-wake-up.htm
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberry-pruning.htm
Have the red black berries, though they are not turning black. Whay? Pat
When blackberries (Rubus ursinus) won't ripen, the reason is likely the redberry mite (Acalitus essigi). Invisible to the naked eye, redberry mites live inside of blackberries, which they turn into their own personal diners. The mites prevent the berries from ripening. Redberry mites infest all varieties of blackberries, but do the most damage in late-maturing varieties.
https://www.gardeningknowhow.com/edible/fruits/blackberries/redberry-mite-damage.htm
I have new canes that are 5 feet tall already, should I cut them to 24" now or wait till early spring? They are just long tall empty canes and will not bear fruit this year.
This article should help
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberry-pruning.htm
In past years, my Chester Thornless Blackberries ripen from a red color to black, when they are ready to eat. This year they're going from red color to a shriveled light brown. Probably 90% of the berries have this problem. Are they not getting enough water? And, how much should I be watering the vines?
YOur plant has white druplet disorder. This article will help:
https://www.gardeningknowhow.com/edible/fruits/raspberry/white-drupelet-syndrome.htm
This article may help
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberries-not-fruiting.htm
We planted a blackberry bush last Spring. It grew over the summer and, this year, is showing many berries forming. At the same time, there is a sizeable stalk, growing vertically from the base of the plant. It is just a stalk with leaves branching out - no berries. Should this stalk be left to grow or pruned because it is drawing water and nutrients away from the berries?
Sounds like it may be a sucker stem and you can remove it as it is taking energy away from the fruit production.