Blood for Blood in the hands of a DRK using it correctly could be good. Of course, there is the chance of some derp killing his or her party because of it, but I don't think that should prevent us from discussing it's potential application if SE were to go this drastic.
As an MT, if paired with a cooldown (Shadowskin or Dark Mind for their shorter cooldowns most likely) the 25% increase to incoming damage could mostly be mitigated, and it would cut the DRK's damage reduction of Grit in half, acting as a pseudo Unchained. The MT would hit harder, getting more hate at the opening where they aren't taking super big damage most of the time which could allow for further use of no Grit heroics if needed. As an OT it would go without saying that it's just a flat damage buff, and an OT not taking hits from the boss would only get raid wide damage, which they would shrug off compared to a DPS using BFB so it would work fine. Of course, there's also the risky "MT with no Grit using BFB for fat deeps" option, but that's probably only going to be used by the super skilled groups or again, the derps asking to die in DF.
I would hate to lose Foresight though with their slight deficiency to physical damage, so what I'd actually do personally is this: make LNC/MRD their cross class, and give DRK their own version of Provoke. Even though it'd be ideal for them to have access to the proposed skill as early as a GLA or MRD would and therefore change an existing skill to have Provoke's ability (perhaps making Abyssal Drain their pull and obtained earlier and make Unmend their Provoke), one could also argue that Voke doesn't become crucial until 50 and up. So what they could do as a more "simple" fix is make a Dark Arts modifier for Unmend that has the Provoke effect in addition to it's own hate generation, making it so the DRK would have to be aware of their MP before a swap (further reinforcing SE's intended playstyle of MP management on DRK) but in return always has access to a Voke if they have the MP.
This is a very good idea, even if SE is so adamant against allowing blocks or parry to spells. Paired with Dark Dance and especially with Keen Flurry if we were going by the original poster's proposal, DRK would be a force to be reckoned with in dealing with magical attacks.
Also a very good point. Hmm, perhaps add the effect of Convalescence to Walking Dead? Or perhaps a Dark Arts version of Shadowskin that includes the effect.
EDIT: Thinking further on it, what they could also possibly do to address the need for a Conva equivalent is add a brief increased healing effect to DASE in Grit, since it's used often by a DRK anyways. Like an "increased healing buff" version of Inner Beast, and again tied to MP use.