Mastery gems - Assuming you know about mastery, but just in case; Mastery for Blood DKs increases the size of your Blood Shield, which is dependent on the size of the heal you receive from your Death Strike. If you Death Strike after a 50k hit, you'll get a heal for at least 15k, and a Blood Shield that corresponds to the amount of Mastery you have. At enough Mastery for 100% shield, you'll get a 15k shield that will absorb that much damage, acting as a kind of "block". Increasing your Mastery increases the size of the shield, which can go beyond 100%, to increase the amount of damage that can be absorbed.
Stacking Mastery seems to be the way to go. If you decide to go this route, note that you HAVE to be using your Death Strike often enough to make use of your Mastery, so if this is not you, or you just have a lot of trouble using Death Strike for some reason, then you probably would favor avoidance (dodge/parry).
Gemming for Dodge/Parry instead of Mastery is not as good, but probably works well. If you don't know how this side of avoidance works, with diminishing returns and all that, then I'm not sure what to tell you. Having dodge/parry keeps you from taking the damage you would have otherwise taken because the boss missed, or you were awesome and slapped that dragon's tooth away. *shrug*
Dodge/Parry is not something you have to actively worry about. It is there without any input from you, taking that whole part of your survivability out of your hands. If you like the idea that you don't have to think about your mitigation, then this might work better for your playstyle. There have been people that have reported it works better on tanking adds - I have no experience here so I'll leave the decision to you.
What you should not do is try and balance them. Then you get the benefit from neither side, and actually take more damage as a result. There are equations and explanations that I could bring in here but I'm not going to - if you want to read up on it there are discussions over at EJ and pwnwear.
I'm sorry my first response was not satisfactory, but I assumed you would decide based on what felt better to you, and what you respond to "but if you are not really good with your Death Strikes, then avoidance will probably help you out more." This was meant to give the choice to you.
If you don't want to have to decide, then I'll tell you right out: go straight Mastery. Now you don't have to figure out how skilled you are or what feels better to you.
Enchants are the same way - go for as much Mastery as possible, where you can't, stamina and armor work in your favor. Reforging is the same way. Reforge for mastery. Hit and Expertise will probably go first, then parry. Dodge dead last if you can help it.
Heart of Thunder is an ok trinket - but its not amazing. There is the mastery Tol Barad trinket that has a resistance use, it is considered one of the better mastery trinkets at the moment.
Your spec is fine, but if you want another tanking CD, spec into Lichborne.
I like quoting myself..There are a lot of DK tanks that do not use Lichborne, which is fine, but if you are looking for another survivability cooldown, this spec - http://www.wowhead.com/talent#jcGr0sMruszG0obZ0b:so0zk which is made for 10 man (interrupts needed) gives you all the survival points. The Lichborne macro is as follows:
/cast [@player] Death Coil
So that when you use it, it Casts Lichborne, and then Death Coil on yourself for the healing benefit DC gives to Undead. To get the full benefit from this, you'd need the DC glyph, and to be at 130 RP ahead of time. It is a very planned CD, so it takes a bit of practice to master. Using VB at the same time can also greatly increase the healing from this CD.
Anyway, hope this helps.