The problem:
Dark Passenger is not useless, but its darn near useless.
You'd see practically no difference in performance when its taken completely off your hotbar.
(The most you could ever see from it, is its Blind effect for trash pulls, and even then, we still have DA+AD to help the healer a tad.)
DRK solely relies on larger trash pulls to keep its mitigation available, and if undergeared, or the healer is too, then you're at a high risk of wiping the group.
For bosses, we have TBN for tank busters, so we can use our normal mitigation for cleaves/filler auto attacks.
But now WAR has a base 8% aoe shield (i think 9.6% in defiance) and as long as they popped another ability before, and wait till the last second of its duration, its a 12% (+14.4%)
This is on a long CD, and acts as a mitigation in itself for the WAR.
Its not ground breaking, but with WARs already ease of use for the group, in comparison to DRK, DRK needs a slight buff.
(Holgang lining up better for TBs, generating more threat in DPS stances, not losing out on as much DPS when using a threat combo, where DRK loses out on DPS and MP for more DPS, besides blood/beast gauge, AoE mitigation, rather than MT mitigation, which is usually useless, as a group with 2 WARs can mitigate all TBs just fine w/o TBN on them, so its not really helpful. Plus the weird 6 potency loss using TBN, then when your group has no SAM or NIN, our raid groups DRK had to switch to their lesser geared WAR, to raise slashing DPS, since its a DPS increase, ease of stance dancing, and then WARs higher DPS over DRK. *breaths*)
TL;DR
My suggestion:
Dark Passenger remains the same potency, and MP cost, like you already want SE.
But now place the blind effect into the base form of Dark Passenger.
Then, when under the effect of Dark Arts, Dark Passenger now places a "-10% damage dealt" debuff on the enemies.
There will be almost no DRK who feels this ability is now useless.
Its a costly one, that is for emergencies.
Raid needs extra survivability, u sacrifice your MP on a single target, but help them survive. You gain no blood like you would on TBN.
Need a filler mitigation tool for trash mobs, because ur undergeared, and 2 groups is all u can handle?
Since this acts as a sort of mitigation CD, the CD of DP might need to be adjusted. (but its so MP costly, maybe not really, as it might get treated the same as WARs old 3.x Storms Path, as most WARs didnt use it, as it cost them DPS, except when it was absolutely needed.)
EDIT: Another point to bring up, is that everyone who is against this change, isnt against changing DP, just that these changes just arent enough, and are still making DRK too clunky, with near useless abilities. (Like plunge being slower than just sprinting, and almost useless for push backs like exdeath).