It depends entirely on who you ask! If you ask a physicist, an artist, and a digital designer, you will get three completely different answers. Here is the breakdown of why black plays hard to get in the colour world:
The physics answer
In science, colour is all about light. When you see a red apple, it's because the apple is absorbing every other wavelength of light and reflecting red back to your eyes. True black occurs when an object absorbs all visible wavelengths of light and reflects nothing back. Because it reflects no light, physics says black isn't a colour—it's the complete absence of it.
Art & chemistry answer
If you are painting, things flip completely.
As a pigment: To create black on a canvas, you have to mix colours together (like cyan, magenta, and yellow). Because you have to physically add pigments together to create it, artists absolutely consider black a colour.
As a technical term: In colour theory, a shade is specifically defined as any pure colour mixed with black to make it darker (e.g., navy blue is a shade of blue). So while black is used to make shades, calling black itself a "shade" is technically a bit of a loose definition—though people use it that way all the time.
Digital answer
On your phone or computer screen, color is additive (RGB). Your screen creates colors by mixing Red, Green, and Blue light. To get black, the screen turns those pixels completely off. In the digital realm, black is a specific value representing zero light.
Quick Cheat Sheet:
In light (physics), black is the absence of colour.
In ink/paint (art), black is a colour.
In vocabulary, people call it a shade, but it's actually the thing that creates shades.
