I don't understand what I am wrong with this blend layer ?

Attached 2 picture, the file is black backgroud, with a blend layer

1. Why the blend layer changed color to blue (pic 1 + pic 2)

2. I think the black backgroud make final effect too dark, then changed from black to deep blue, the applied blend layer, you can see original (black + blend) is very smooth gradient color. BUT my work (deep blue + blend) showing some lines (not smooth) (pic 2)

Why ? Please help