Buffing blood shield a bit wouldn't do much at all for the spec, you don't really ever die when you can use death strike especially when taking high damage (which is the case that would make blood shield relevant), you struggle when you've been forced to use death strike and you run out of rp (and when that happens you've to choose beetween rebuilding bone shields and building rp fast to even stay alive)
My experience has been way more frustrating on lowish damage content, you don't have control over your survivability at all because your death strike doesn't heal for shit, it just doesn't feel fair to death strike for 400-500k when getting meleed for 700k
Everything just feels undertuned unless you're taking an absurd amount of damage (which you will on Ursoc and Xavius) and then you'll just yoyo really hard, make your healers sweat and have to play flawlessly to get things done (when a warrior can accomplish the same by pressing devastate and shield slam, refresh shield block and ignore pain when needed)
Here are the changes i'd suggest to make Dks even relevant in raids:
-Revert bone shield back to 20%, fix dots removing blood shield charges (seriously)
-Make Death Strike heal for 20% of max hp minimum, 10% is just not enough, on lowish damage content not only dks have no mitigation, but their selfhealing is bad(if you don't believe me just get rid of bone shield/blood mirror in your healing logs because it's not actual healing and see how dks fare against warriors or dhs, classes that have actual mitigation on top of that), self sustainability was always a dk niche and now it's just gone
-Double Ams absorb, it's way undertuned, on Xavius it literally absorbs 1 and a half tick of the dot that ticks every second
-Give vampiric blood 2 charges, if you're not gonna give Dks dr because of "class fantasy", they still need more cds that actually work (dancing rune weapon is parry which is irrelevant for 99% of tank mechanics)
-Buff rune tap to be a 40% dr: it's a niche talent you're only taking when you really need it, and when you need it 25% dr just feels like a meme, it's not enough and you've to compensate the fact it's not enough by calling externals at the same time you're using rune tap, which is exactly the opposite of what the talent is supposed to accomplish
I don't even think these changes would make dks great tanks, just not a liability to their raids like they are right now.