Like the sibling comment said, 'red pill' is seeing the truth of the world (or, what the community that uses the term red pill thinks is the truth at least. often this refers to the truth as decided by internet mysoginiats). Black pill is this truth + its unchangeable and hopeless and pointless and why do anything and oh god I wish I could go back to not knowing.
If its used by somebody reasonable, blackpill is a pejorative with the implication that the ideology is more cynical and hopeless than it really should be and has the connotation that their ideology is a little too internety.
I think of "black pill" as a GMO'd red pill designed to nudge the "learner" into a position of learned-helplessness.
Take a common aphorism around here: "99% of startups fail, so you shouldn't bother trying". The red pill is "failure is the most common modality for new ventures", but the attached advice makes it a black pill.
In a meta way, the linking of the term "red pill" to misogyny/misandry turned the red pill concept itself into a black pill (why bother seeking "red pills" if the "community" most identified with them is so disturbing?).