|Japanese blueberries tops dying back|
A. I would guess that something caused a problem on the trunk or large limbs at the bottom where it has turned brown. I am guessing it might be borers or some other critter that mechanically damaged the trunk/limbs. It is possible it could be sunburn as well particularly if they were open enough to allow direct sunlight on those branches/trunk for extended periods of time.
I think we can rule out diseases in our climate except for the possibility of sunburn (non pathological disease).
Look at the area of trunk/large limbs just in the foot above the healthy areas and let me know if you see peeling bark, discoloration or other types of abnormalities.
If it is borers (the most likely), there is nothing you can do to raise it from the dead but you can protect it with a soil drench once a year using Feri-Lome’s soil drench. It contains an insecticide that should give you about one year of control from what I am hearing.