Cort Ammon mentions the thickness of metal needed to act as a mirror, which seems to be the right value for the penetration depth of the electric field of visible light frequencies.
But graphene is completely different. A single atom-thin layer will block πα ≈ 2.3% of light.
Now a blade, to be useful, will not be some ordinary molecule. We expect it will have extaordinary properties and graphine is a good example of what we might expect. It might be unusually opaque, reflective, or invisible compared to what you expect of normal material that thin; and the details can vary with the frequency of light, producing complex color effects.
Note that unusually opaque is still only 2.3%, and unusually reflective would be the same order of magnitude. Most light that passes by simply does not interact with the electrons at all.
Now is the blade thin in one dimension or two? If it’s a nano-wire, say a superconductive carbon nanotube given rigidity via magnetic fields, light will simply refract around the wire that’s smaller than a wavelength of light.