it's actually answered here: http://harem-battle.club/dmm-version...mm-advice.html
but the idea is that just choose the black that you want to get the most. there's no ideal black to get aside from waifu because different units are better under different situations. I could choose a healer unit but it doesnt mean shit if I don't have a tank who can survive long enough for the healer to heal, or I can let my units survive but end up lacking dps/blockers so I leak monsters.