Unlike other flat, zero-width textures in the game - such as cross models like grass, flowers, and kelp - the stonecutter's blade doesn't have the "noshade" flag set which causes it to render unusually dark, giving it the appearance of being made out of stone rather than iron or steel (which it's actually made of).

I've added comparison images below of the flag as it is in vanilla and a resource pack I made which adds the flag; the difference is quite striking.