This depends on the display technology used by your mobile phone.

AMOLED display is a type of display that consumes less power when dark wallpaper / theme is set. It also usually provides superior color rendering properties compared to liquid crystal displays. WallpaperNext Black wallpapers

AMOLED stands for ActiveMatrix Organic Light Emitting Diode. AMOLED screens are made of organic materials that produce light when an electric current flows (“electroluminescence”), allowing the AMOLED display to light up pixel by pixel. Each pixel emits its own light.

This means that unlike a liquid crystal display, if you want a particular part of the screen to appear black, no current will flow through the pixels in that part. AMOLED displays, which are much newer than

LCD displays, are also more expensive to manufacture.