if you want to prune BB, do that now...no harm to cut back on the non blooming branches, i do that with mine to take some of the legginess of the plant away...for pruning the flowerheads or the branches, i always prune it down to the next set of healthy leaf buds...so the leaf buds and very short stem stump (about 1 cm) will become the terminal end of that branch...as for reblooming...if you do the flower head prune now, you should see the leaf buds to grow vigorously - assuming we still have some summer left for this crazy season in the NE - and flower buds could well develop by end of this month, and you will get blooms in mid-Sept or so...so go down the branch, cut right above where you see a set of healthy leaf buds...if there are multiple sets of buds, I either chose the healthiest ones for location or take the shape of the whole plant into consideration...BB is really a strong performer in terms of blooms...just it has that tendency to become leggy with age...like my BB - one of the few hydrangeas to keep on sending out fresh blooms even in Oct here in New Jersey (if you prune the flowerheads in July/early Aug)..from late May/early June till end of Oct - that's a very long flowering season for here...almost like the million bells annuals..