Blackpillers are men who believe in blackpill philosophy . As the name suggests, the label was invented to provide an alternative to redpill ideology, The original definition of the black pill remained obscure, and was only really used by a few people who vised Omega Virgin Revolts/ blog. The term started getting used by nihilistic red pillers, who wanted to give up hope, because they could not get a girl.
Modern blackpillers are generally misandric, as they spend inordinate amounts of time status-shaming other men, and/or calling them subhuman.
Also black pillers seem to be obsessed with women, and think one should give up on life if they can't get the girl.
Later on it was adopted by youtubers like Eggman who used the term ,,take the Black Pill". Unfortunately at this point entire idea turned into Radical Lookism, Self-loathing and defeatism. It simply nailed down everything to men looks and operated on belief that attractive men are really loved by women, are never cheated on or discriminated against. Also similarly to PUA (which was the enemy of original ideas) it completely ignored importance of resources and status of a man in Dating. It became simple biological reductionism
If you have a good genes and you are attractive all women will respect you, value you and you will get sex for free - women will cheat on men with status with you all the time while they work their ass off to pander to them. If you have bad genes and you are ugly women will be disgusted by you and you will become a target of feminism, you will be cheated on and you are better off becoming off grid hikikomori.
It is just that. Nothing more. If you want modern version of this there are youtube channels like LMS who still believe in this nonsense.
As most failed ideologies, similarly to Redpill, Mgtow and Purple Pill this became taken over by tradcons who provided same solution as to everything - arranged Marriage. Some are even said to go as far to believe TFM or hating Incels.
Therefore BlackPill is a good idea of very honest and aware person that turned into crap once it become distorted.