Black does not exist.Accept this truth, you must, if to understand art you want. There is no black pigment. There is albedonote , color, and lack of albedo and color. No black.
Most experienced artists will happily explain that the only thing that is actually black is a black hole and that even at the bottom of a sealed mineshaft you will probably find some faint phosphorescence. Even black pigments are not black but dark grey. And yes, pigments. plural. Black comes in different colors. There are three charcoal blacks, six standard black printing ink mixtures, and dozens of off-black pigments. A standard HB (#2) graphite pencil has an HSV value of approximately 50% black- also known as middle grey, and a high quality black inking pen tops out at about 60-70% black. If you're reading this on a really good monitor with zero glarenote , this rectangle: █ is probably a 95% black. A more typical monitor in average lighting? 80%-90% black.
Every physical object that is not a black hole reflects light to some degree. That's how we can see things. The closest thing to pure black pigment is the nanoparticle coating Vantablack invented in 2014, and the images in that article demonstrate perfectly what it would look like if black things actually looked black: that's not a pile of vantablack sitting on aluminum foil, nor is it a real life portable hole. That is crumpled aluminum foil coated in vantablack. There are no depth cues, highlights, shadows, or reflections. Just a flat meaningless black blob that might be making your brain freak out a little bit just looking at it, and probably has a goth or two squeeing because someone actually made a color darker than their favorite outfit. No light, no form, no image. That'd make for pretty boring shadow magic.
Thankfully, most objects not coated in physics bending nanomaterial do not absorb all of the light that hits them. This allows artists to use Necessary Weasels like exaggerating ambient lighting and reflections to make the form of a black object understandable.
Additionally, the choice of purple isn't arbitrary- it's actually rooted in the science of physics and human perception:
Logic behind this trope
Less energetic frequencies of light are absorbed far more easily than more energetic frequencies: a dark surface will reflect more blues and violets than reds and yellows, giving it at least a slight grey-blue tint compared to the light source. There is also the phenomenon of structural coloration: some wavelengths of light can actually be filtered and reflected by molecular structures, leading to Iridescence. A peacock's feathers are an ideal example of a very dark material which reflects a few shades of color particularly well- the feathers are actually dark brown. Similarly, many birds we consider to be black are actually quite brightly colored in the ultraviolet range, which birds can perceive. Many materials actually do the exact same thing, but we only notice it when the material is a dark color.
Shadows also usually have a slightly blue or violet tint because the more energetic hues of light tend to bounce around a lot longer, giving the air a blue tinge for the same reason that Water Is Blue. It isn't just the sky that is blue; the air around us has a very slight blue glow from scattered light! As with iridescence, we only notice this when there is no other light source to overwhelm this much fainter Diffuse sky radiation. Any light source also creates a diffuse glow around it that filters anything behind it with a tint of that color. This means that unless an artist is invoking Chiaroscuro with hyper-exaggerated shadows, black is almost never used in realistic artwork! What the viewer sees as black in highly realistic images is usually a dark grey-purple/blue, or a very faint tint of the light source's color.
Lastly, there is the color constancy effect, the ultimate Mind Screw of painting: Colors aren't always the colors they are. A pale tone of a color next to an intense tone of that same color tends to look like the complementary color. That purple object might just be pale yellow- it only looks purple because it's next to an intense yellow. Since lighting is almost always yellow, this tends to make all dark pale things look purple. Conversely, by making the darkest parts of an image blue-purple, you automatically imply that the brightest parts are orange-yellow and the brain will do the same effect in reverse.
To sum up: there is no such thing as black, when things are black they are usually blue or purple, the air is actually glowing blue-purple around you right now, things that look dark purple might actually be pale yellow, and things that look bright yellow might actually be light purple.
Begin to understand why artists go mad?