B. nigra is a hardy, running type bamboo of moderate height and vertical growth. It is best to dig, divide and transplant at a time when tempertures are not extreme--ie. avoid hot summer or freezing winter conditions. Late winter or early spring would be optimal. Use standard good transplanting practices: dig straight down with a long sharp spade around the perimeter of the plant, digging a small deep trench around it, preserve root integrity as best as you can, avoid exposing roots to drying winds, and move to the new, previously prepared site as expediently as possible. Cut back tops somewhat to compensate for root loss.
March and September are optimal, other times suitable if an Arctic Front doesn't blow in right after. Dehydration is the main enemy, if roots and tops are kept from drying out success is likely. Intact tops probably promote root growth, as with broadleaf plants, so cutting back the top should only be done for convenience in handling or where the whole culm will break off of the disturbed root if left to its full height (try to get solid, intact rootballs).