I am just coming back to WoW after a brief break and have a few questions. With the gear from the Timeless Isles, and a few earlier lfr/crafted/JP gear could one realistically, and successfully tank the LFR version of Siege of Orgrimmar? Also, I have seen many blood dk's gem and reforge for avoidance, is this to be done at higher gear levels? or are the avoidance stats now superior to mastery? I currently use mastery as my main reforge but want to be sure. Thanks for your time.
If you have the minimum itemlevel to get into LFR, you can complete it. It may be difficult at 496 itemlevel, but it's certainly doable. Personally I have been running LFR and Flex SoO on my blood DK in my full dps gear and outright stomping it, and I've gone from itemlevel 535 to 545. If you put in the time, the bosses will fall eventually (to Determination stacks). If you have any inkling of how to play blood DK, you should do just fine.
Mastery will give you the best return if pure survival is what you want, but avoidance currently gives some extra damage due to riposte. Gear how you feel comfortable. Typically you'll want higher survivability at lower gear levels.