Hope this helps!
ok, i've been DK tanking for a while now, from level 55 i knew i wanted to be a DK tank (before i even made the toon even lol) though i tank with frost talent tree, opposed to your blood tank spec... based on a lot of what i've read, there are a few flaws with your gems and sigil.
i'd say the gemmed expertise is a bad idea as a tank, though it improves your threat, it's not much of a difference. Focus on stamina, same with the def rating gem, toss it.
another thing, obviously, the sigil is not exactly helping your avoidance, considering, at least according to wow-heroes.com, you have an item level 245 dps sigil equipped (Sigil of Virulence) try to get Sigil of Insolence (item level 245, effect: whenever you use Rune Strike, you have a chance to gain 200 dodge rating for 20 seconds) or Sigil of Deflection (item level 226, effect: Rune Strike grants 136 dodge rating for 5 seconds) or, if you're not def capped, there is also Sigil of the Unfaltering Knight (item level 200, effect: Your Icy Touch will also grant you 53 defense rating)
The World of Warcraft Armory
now, i do agree with what most of the people are saying, blame the healer, not yourself, a motto i heard of not too long ago, from a guildy.
i'll answer any questions any of you other DK tanks out their might have, or at least try!