The hate is irrational, but the REASON for the hate is that the song that made her famous is...well about the most objectively terrible, horribly executed song out there. You can hate pop music, for example, but it still largely succeeds at being pop music. Friday fails at everything. Putting aside the singing ability, the lyrics boggle the mind, and weird bits of autotune are terrible. Like, not terrible because Autotune is terrible, but terrible at being autotune. Personally, I think its reasonable to say that an adequate singer is beneath all the trash, but the fact that such a horrible song is layered over the top is downright embarrassing. Bad enough to color all future perceptions of the singer. Its like how Mark Hamil was always Luke Skywalker, Black will always be the girl who made the horrible, "Friday". She could make the next "Stairway to Heaven", and she will still be remembered for "Friday".
Why does "Bad singer" turn to "hated"? Well half of it is because people take there music way too seriously, and the other half is that complaining about pop music gives indie cred within pretty much any genre of music fandom.