Blood Tank Reforging

Personally, I'd wait until you start gearing at 85 before reforging.  I noticed during leveling from 80 to 85 that I was swapping gear out constantly, weapons, shoulders, everything just seemed to be constantly replaced.  This is expected, so don't worry about min/maxing just yet.  Although, the cost is low enough, if you are struggling with tanking some dungeons, go ahead, throw a reforge on there to get used to it.

Mastery is a great stat for Blood tanks and generally speaking this would be the stat I would reforge into.  However, you must first be to the Hit and Expertise soft cap.  This will become an issue as you struggle more and more for aggro as your DPS starts gearing and becoming more of an issue.  After hitting the mob is no longer a worry, start putting some points into Mastery.

A Death Knight Tanks Mastery Rating will affect Blood Shield.  Blood Shield will proc off of your use of Death Strike, which will provide a 50% damage absorption of what you were healed for.  The percentage of damage absorption is directly affected by how many points you have in Mastery.  Each point in Mastery stats will increase the percentage of absorption by 6.25%

More Blood Shield information on Wowhead.com.

Death Knight Tank Reforge Gear Guide

With the introduction of 4.0.1, everyone has access to an NPC that will reforge your gear.  That is, reforging will allow you to take 40% of one stat and convert it to another stat not found on that particular piece of gear.  Reforging cannot affect gem slots or base stats, which means as a Death Knight tank you will typically only be attempting to reforge stats to reach cap on hit or expertise.

When choosing a stat to reforge, consider stealing from hit or expertise.  The reason for choosing these stats is because you are most likely over the cap for one or the other, if geared well.  Take the amount over cap and convert that stat into the one needed or into the new Mastery stat.

Basically the reforging process is a method to get rid of useless over cap stats and convert them into needed stats to reach cap.  Once caps are reached, spend excess stat points on the Mastery skill.