Ngl this is hella convoluted
First of all, what does "naturally" dark even mean when talking about forging swords from metals? And not like the pictured sword was even forged but more likely cast, meaning it's likely a mixture of more metals hence the color.
Second, why would Oda draw a blackish blade without it straight up being a Black blade? Makes no sense. Like "yeah, this blade has a blackish hue but only because it's a Naruto cosplay Uchiha Sasuke grass cutter"