Shield Slam gains extra damage from Block Value. The amount of damage your shield absorbs with one block is modified by your Block Value. While the tooltip says "Increases the amount of damage absorbed by your shield by 10/20/30%." it really means "Increases your Block Value by 10/20/30%.".
While this, from one point of view, is exactly the same thing (due to block value being the number for the amount of damage blocked), it's slightly different when you factor in Shield Slam. Mainly because that too is affected by Block value, only damage-wise instead of absorption-wise.
The answer is simply: Yes, Shield Mastery does increase the damage of Shield Slam. Anyone arguing for the opposite has no idea about anything, least of all game mechanics.