Visible light is the visible spectrum of electromagnetic radiation which upon contact with the sensitive medium can be cognized by the nervous system.
When there is no color vision, that doesn't mean that the eye has no object, there is still vision but it is without color.
When you decide to look and see 'darkness', that experience depends on the nervous system interpreting, conceiving & perceiving the conditions that can be known with the eye element.
If you had no eye then looking wouldn't occur and no information would come into being based on the eye-instrument.
The absense of color vision is seen as a darkness with the eye element, that which sees is the eye, the uncolored spectrum visible is the seen and consciousness cognizes it as 'dark', perception perceives it as 'darkness', can be felt as pleasant or otherwise depending on the circumstance.
This directing of attention to seeing gives you knowledge you wouldn't have otherwise.
A person with no eye can't see darkness in a dark room, can't perceive shades or even direct attention to that knowledge & vision of darkness.
It is kind of evident in that you can see dark spaces in an otherwise illuminated room and there darkness is something you see & know with the eye due to seeing.
You can also think about it as seeing shades of black color as more or less uniform in terms of a distinguishable shape.