I wouldn't mind if oblation's buffs were put onto the blackest night instead of having it as a separate skill but I do remember seeing a post that brought up some good points regarding it and I do like the fact that you could throw out oblation on two party members and tbn on either yourself or the main tank.

Something I think would go a long way would be reducing the mp cost for tbn
That or increasing how much mp you gain with your combo and skills including carve and spit
Could even have an additional charge to the skill so that instead of picking between healing or getting mp you could do both, or you could heal twice with it or regain mp twice

Granted my dark knight is only level 92, so perhaps they gain additional charges later. I'll have to read traits I unlock later.
I do really like shadow vigil, it looks pretty cool and it seems to also have a healing effect when it expires or if you go below half health kind of like heart of corundum