I'm hammering down with a fury of Thor himself on that poor battered plate armour I found, but it doesn't seem to get any boosted abilities.
The upgrade window shows "Boosted Plate Armour" but nothing happened.
Oh and why do I need crafting skills instead of blacksmithing?

It's a combination of your skill level and the item you're trying to upgrade. It won't upgrade the stats if you're level isn't high enough..I'd check the durability and the speed on the armor itself. Crafting is for the creating of items themselves, while blacksmithing is for upgrading If you don't have crafting you cannot even make any armors.