If a tank loses aggro, whose fault is it?

[JunseiKei]

~snip~

For your particular situation, the tank is completely at fault. The only aggro management you can do as a BLM is use Quelling Strikes whenever it is up.

That being said, it can be a DPS's fault for not using skills that halt or reduce enmity. A good tank shouldn't lose aggro, normally, but that's not accounting for lucky crits or enmity generation potent openers (like bard and machinist that build extremely quickly off the start). However, stopping DPS is never the solution- you may need to switch targets (if in single target rotation).

Likewise, it can also be a healer's fault for over-healing.