Short answer
As with white, the answer depends on the frame. In terms of light, black is the absence of it, so not a colour. In pigment and print it is a manufactured colour with real, differing formulations. In everyday and design use it is unambiguously a colour, and one of the most deliberately chosen.
Black is what you perceive when little or no light reaches the eye from a surface. There is no black wavelength; there is an absence of returned light.
Black pigments are real materials — carbon black, bone black, iron oxides — with different undertones and different behaviour. Printers treat black as a distinct ink, the K in CMYK, precisely because mixing the other three does not produce a satisfactory one.
On a display, black is the absence of output. This is why screens vary so much in black quality: a backlit panel can only dim so far, while a panel that switches pixels off entirely can produce a much deeper black.
Black is chosen, specified and compared as a colour constantly — and distinguished from near-blacks like charcoal and midnight blue, which would be impossible if it carried no colour identity.
The apparent contradiction resolves once you notice that the two answers describe different things. As a perception, black is what happens when a surface returns almost nothing to the eye. As a material, black is something you add: a pigment with a composition, a cost and a character. Carbon black and bone black are not interchangeable, and painters have distinguished between them for centuries. Both statements are true at once because one is about light and the other is about stuff.
Real surfaces always reflect something. Even materials engineered specifically to absorb light — the darkest of which absorb well over 99% of visible light across the spectrum — do not reach zero, and ordinary black paint reflects a great deal more than people expect. This is why blacks differ visibly: one leans blue, another brown, another green, and a black fabric beside a black plastic rarely matches. It is also why photographing black objects well is difficult, since the eye tolerates far more variation in dark tones than a camera does.
The same limit applies in the other direction: no white is perfectly reflective either.
Black is the highest-contrast partner for almost any light colour, which is why it dominates text, signage and interfaces. That strength is also its risk: black absorbs so much light that it flattens the shadows and mouldings that give a room or an object its perceived shape, which is why a black-painted interior often reads as smaller and less detailed rather than simply darker. Designers frequently reach for charcoal or a very dark neutral instead, keeping enough tonal variation for form to remain legible.
Wavelength ranges and perceptual thresholds are given as approximations: both vary between individuals and with viewing conditions, and sources differ on exact boundaries. Where a figure is contested or unavailable, this page says so rather than picking one.