To be honest its pretty bad. Might aswell MLB it for the event, i doubt you need more than 1 pure damage dealer for the 200% damage buff.
Outside event i think its too slow, i would personally never use it. Heavens feel is same effect but instant (40:50% np strength) but harder to mlb, not that 10% dif is much.
I dont really even use normal black grail much unless i want to get into some weird buff stacking np looping one shot stuff. CE that have charge and more instant basic effects are probably better. But i guess its fine if you are doing multi wave quest where your main damage dealer is saving np for final wave, but they also have to be able to build gauge themselves or with teammates.
edit: overall its pretty meh and because of the effect, non mlb is not much different to mlb, since it retains 5% strength each turn, with mlb only increasing max to 50% instead of 40%. So i guess you could just leave it non-limit broken, but its not exactly much use to have 5 of them, it doesnt work for benched members. And i doubt all 3 front line need np strength.