it depends on the grade of steel.
ok, the long explaination....
steel is iron.
when you remove the impurities from iron, through the smelting process, you create steel. low grade steel is darker. it is harder than high grade steel, yet more brittle, good high grade steel is softer, yet will not snap when it is bent, it will, rather, bend and whip back into position when released. ever heard of spring grade steel?
anyway, to be technical, no a katana normally wouldn\'t be that dark, unless it was low quality workmanship. but that doesn\'t mean that it\'s not more aethetically pleasing to some people.
edit: almost forgot, steel, before tempering, can be dyed a darker tint. I had nearly forgotten about gun barrel tinting in the wars(gunmetal blueing technique), also, ninja once dyed their blades so that they would be less noticable in the dark