Would you really expect a knife historically known as "White Hilt" to be all black with an /obsidian/ blade? Similarly the knife used on the Stone table was described as a bronze leaf-shapes blade with a simple wood and leather hilt.
Fwiw, where Carnwennan appears, it is one of Three magic weapons attributed to Arthur: Rhongomiant, Arthur's spear, and Caledfwlch, Arthur's sword, as sacred weapons given to him by God. This has me thinking of KotC. Since we already know that a) Arthur had Amoracchius, b)Wielders of Amoracchius are considered unusual even by KotC standards, and c) the only time a single person has wielded multiple Swords was Michael (DM, Ch. 32). I have to at least raise the possibility that Arthur was just so damn badass that he was actually given use of all three? As far as Hero levels go, he's got to rank up there with Merlin, who himself seems to have all but built the modern world order while breaking nigh cosmic-level natural laws (DR construction)