It would be a new skill entirely, it would not be Oblation. You could use one stack of Oblation, consume it with this hypothetical mitigation, then use your second stack if it was necessary. Let's call this hypothetical ability Occult Acumen (FFXI reference) for clarity sake. This would give DRK the most personal mitigations but (TBN aside) with all of them being pretty weak, I think it's a matter of trading out strength for duration and requiring stacking them since smaller percentages have smaller diminishing returns. So you'd be able to consume Dark Mind and Oblation, with more consumed abilities making the healing effect stronger, with no heal at all if nothing is consumed.