On World of Warcraft's official forum site for Europe, one user named Borodin posted this:

Shield Mastery helps to make Block a stronger source of overall damage mitigation but don't overlook the benefit to Shield Slam damage and threat: with current endgame Tanking gear being loaded with +Block Value, if you have Shield Slam you really should also take 3/3 Shield Mastery.

• Your total Shield Block Value is the Block Value of your Shield + all items or abilities which "increases the Block Value of your Shield by n" + Strength/20 (will edit if I can be shown it's Strength/21or22). This is the amount of Physical damage you absorb on a successful Block, not a change in the ShieldÂ’s Armour.
• If you take Shield Mastery you then gain 10/20/30 % increase to that total value. This therefore includes all items with a +Block Value component not just the value inherent to your Shield. In essence all sources and terms are combined into one simple value: Block Value and Shield Mastery increases that final value.
• Your total Shield Block Value does apply fully to Shield Slam damage. Note that Shield Slam gains your total shield block value as damage not just a proportion of it for being an "instant cast spell". Frankly I think Shield Slam should be reworded to make it clear that you get your total Shield Block Value as a modifier and it should even dynamically display that damage as Bloodthirst does.