Converting from a DPS frost DK to a blood tank
Can it be done?
Well the question is how would I go about obtaining gear since I've used up a majority of the Cata quests...
Should I buy some BoE's, maybe get a tank weapon and just run non-heroics collecting tank gear to replace my dps gear as I go?
There isn't really a tank weapon for Blood right now. Anything with mastery on it is a tank weapon (I would not advise dual wield tanking) with preferably expertise/hit as the other secondary stat.
As for gearing up, depending on your reputation levels you can just get half of a very complete tanking set from rep vendors (neck, cape from hyjals, boots, helm from dragonmaw, wrists from ramkahen, ring from therazane).
Then you could fill in the rest with justice point gear or non-heroic drops or even reforged dps pieces (assuming you have high quality dps gear, anything with mastery can be converted to tank gear in a pinch until you get something more tank-y).
cool thanks for the advice I'll be sure to look into what i can get with my current rep!
Best weapon from heroics is Foehammer, otherwise any weapon with Mastery or Expertise or even Hit Rating is good.