It deposits purple pigment onto blonde hair to cancel out yellow/brassy tones and get it closer to a white. It’s not the best imo. Wella ColorCharm T-14 or T-18 work much better on me, but it’s gentler on the hair and works well as a touch up (even though purple shampoo is probably better).
To dye my hair black, I used the Revlon Colorsilk Soft Black cuz it was cheap. Never did me wrong. If it fades, it just becomes a little more brown. Nothing odd.