I personally use miracle grow on all my canes. Seems to work wonders compared to not using it. I personally would fertilize them at the beginning of spring at leaf out and one more time around full bloom/fruit set.

I also use miracle grow. I mix one tablespoon to a gallon, spray it on the leaves till it drips off, and pour the rest around the crown. I do this every two weeks or so from bud break till Mid-August.

It's worked well for me so far, but I'm no pro.

By the way, I always do this in the early morning or evening around sunrise/sunset. Apparently the leaves are supposed to absorb things better at those times.

Plant leaves wax soft at 77 degrees and above at plant sunshine helps this temperature on wax.
Blackberries taste the best if fertilizing done when no fruit on plant if done while fruit cone druplet form on stay green to long and run flavor berries you check this out a picking time just look point where berry broke from stem green flavor not there gray good tasting fruit.
Timing fertilizer is at 5% bloom and again at 100% bloom this size up all berries and nitrogen be gone before harvest time. Fertilize just before last fruits are picked.