Crushing Blow doesn’t add damage to Hammer Throw. Before putting a point in Crushing Blow, I see my Hammer Throw DPS at 1396. After putting a point in, it still shows my DPS at 1396. Testing with actual hits confirms.
If it matters, I do have Mana Starved Forging, which reduces Hammer Throw’s mana cost to 0 when I’m out of mana. Normally, Hammer Throw costs 15 mana, and so I’d expect 0.5% x 15 = 7.5% increased damage for Hammer Throw.
Also, the [alt] more information for Crushing Blow has a paragraph that only mentions melee and not throwing, making me suspect throwing was added on later but not implemented.