Black Pill is an expression, referring to the taking of the allegorical black pill; not as bright colored as the red and blue in the Matrix movies, but a lot more intense.
Taking the black pill basically refers to the realization to the ugly truth of life; the world is a hostile and unequal place, set in the favor of certain people with certain characteristics; be it sex, skin color, intelligence or looks, all people are born different with different potential.
Origin of the term
Ever since the Matrix, taking a red pill had been associated with awakening to the realities of society and the world, while taking the blue pill had been connected with refusing to see the truth.
Black pill ads a grimmer alternative; the realization of the harsh hostility of reality.
The term is favorably used by the Incel community on the internet, as it also originates from an anti-feminist blog, describing the female desire as inflexible and their psychology is mostly determined by genetics and instincts.
Spread of the term
The black pill had turned into a cultural phenomenon since then, with lots of doomers seeing life as a hostile process, determined to crush people or make them ascend from their difficulties.
It is used widely by people with alt-right views, as well as the aforementioned incels, but it is a philosophy, that strongly resonates with the doomer world view; one perceiving the world as a hostile, unintelligible place.