Block is more difficult to model and depends on spec, gear etc. The good thing is that since you are a warrior you don't need to worry about it too much! Due to the fact that block reduces incoming damage by a set amount it can be great against trash (mitigating a high % of their low-damage attacks), but it does less against a hard hitting boss (reducing his blows by only a small fraction). When it comes to gearing you should pretty much ignore it. If there are other items of similar qualities you should prioritse expertise > hit > dodge > parry > block (assuming you are not hit capped).
As much as you can get without sacraficing Effective health (armour and stamina. but the amount of avoidance you need will vary greatly depending on what you are tanking.
For heroics Block value and block rating are kinda powerful, getting your BV up around 2500, and block to ~40% and you can send your decently geared healer off to make the tea while you clear trash, and he'll be bored by the end of the run. However when raiding; ignore block value. you'll aquire enough gear with strength on that block will be around 1k, and block rating is pants. dirty smelly pants. for avoidance I don't gem for it, don't gear for it. The old rule of thumb i played with was twice as much dodge rating as parry rating, but with the 3.2. you want about 40ish avoidance from dodge and parry I reckon. but really its after thought stats.