Here, three primary subtractive coloured inks have been printed onto paper.
Each of these colours has one of the additive primaries subtracted so, where two overlap, two primary colours are subtracted leaving only one of the primaries visible.
Once all three subtractive colours overlap then all three components of white light are absorbed; no light escapes and black is perceived.
In practice a small amount of light does get relected and a true black is not achievable using only Cyan, Magenta, and Yellow.
To correct this printers use a fourth ink, Black, to create deep shadow areas necessary for good colour-image reproduction.