1) Your friend is a douche

2) Your black mages arent paying any attention to their enmity generation, not your fault.

3) Any DPS that doesnt know how to hold back or to attack the current marked target is going to pull hate off you. That's pure and simple fact that due to only having Flash as your only viable AoE, anything you're not currently also hitting with your sword isnt going to have a lot of enmity on it. Again, not your fault, it's a flaw with GLD/PLD and the biggest gripe of anyone who plays it. It's great on single targets, terrible on multi-targets. Outside of Shield Oath being broken in 2.3 and not working correctly anymore...

Solution: Get nicer friends, let the BLM's die a few times till they understand they shouldnt attack a target with very little enmity on it. You'll be happy, the BLM's will have learned something, and everyone's dungeons will go a lot smoother afterwards.