Thinking about it a little more, to extend my theory reasoning, blackmetal is put in the blast furnace. Going by the name blast furnace I assume it generates more heat and/or at a faster rate? Because this metal needs more heat to refine, its elemental makeup and origin is unkown, I assume it probably ends up more brittle or easier to fracture that's why blades are made from it. A pickaxe, mace or armour would just crack and break apart with in the first few strikes.