Not all of what Prince says should be taken at face value. I listen to some tracks, it don't seem dark in general. Maybe in Prince's mind it was...

All you have to do is play it back to back with Lovesexy to see what he means by good album and evil album.

Black Album's tone is macho, sexual without love, dark musical tones, violent and insane hints within the lyrics. The arrangment is dense thick funk with pools of reverb on some of the vocals (or distortion) that gives it a midnight, impersonal, distancing feel. Something like "Rockhard" seems sinful in the sense that it's 5am after Mardi Gra and people are fucked up on the streets in New Orleans kind of sinful.

There is no cussing on Lovesexy, is there? At least you could understand that difference!

Up until that point there was no overt cussing on any Prince album within the main body of the song. The worst offender was "Erotic City" which could have been a mini black album unto itself.