A competent black mage with decent support will pump out alot more nukes than a SMN will be able to BP: Rage in the same time, which effectively leads to more damage overall.
Only aspect SMN beats BLM in is enmity during endgame scenarios.
With enough summoning magic skill (335) Summoner can be more effective at landing unresisted magic damage on HNM's than the best black mages. Of course 5/5 on group 2 merits is a must. Not even the best black mages can break 1300 without the help of Dark for Absorb-INT.