Tough to say. Perhaps too much or too little water? Stick your finger in the soil about 2 inches down to check the moisture level. Someone with more knowledge of blackberries may be able to give more pointed guidance.

Your plant doesn't look that bad, it could be a water issue as Jason suggests or is that the first cold is setting in and the plant will soon go dormant. If you have room to spare I would put in in the ground, most blackberry are very vigorous and are hard to tame in pots.

The canes that grew this year will fruit next year. Cut them down after you have harvested the last berries, and leave the new canes which will fruit in 2018.

The current 8G pot is plenty large enough to overwinter a young caneberry plant. I would plant it in the ground next March, before any new growth. The leaf browning and dying tip may be lack of water--it can be hard to get potted soil watered evenly, most tends to run down the pot sides--even rain water.

Your primocanes that grew this year are next year's florocanes. It is likely the plant will stay in a clump and not do much running.