Save or not?
It depends on the type of display of the smartphone, namely - from the backlight used in it. In all sorts of varieties of the TFT, including TN and IPS, there is a general constant illumination, that is, each pixel is always highlighted, regardless of its color. Accordingly, even if the pixel displays black, he still shines. With these kinds of display setting black wallpaper will not help, but the reduction or automatic (adaptive) adjustment of brightness can save battery power.
Quite different working OLED-displays. They do not have the same constant illumination, each pixel illuminates itself. you know from physics course that black - it is the absence of light, so in the OLED-displays to display a black pixel is simply switched off. When a pixel is off, it does not consume energy. Accordingly, the more black pixels on OLED-display, the less it consumes battery power.
Always On Display Mode, which is so active PR Samsung and LG in their products, just a combination of black display and minimum brightness, which allows continuously display the basic information without appreciable damage to the charge battery.
How do I find the type of display of the smartphone
We'll have to connect all your skills to find information on the Web and zaguglit request form "[smartphone model] display type."
Now OLED-displays are already quite popular. They are used in almost all models of Samsung and many smartphones from other manufacturers. It is noteworthy that even Apple is planning to transfer all future iPhone on OLED-displays in 2019.
How to make and install the black wallpaper
In any sane image editor you create PNG-file with a solid color black. The height of the images should be equal to the number of display pixels vertically smartphone. Width - twice the number of display pixels horizontally.
For example, if the display resolution of 720 × 1280 pixels and uses a standard launcher to five screens, the resolution wallpaper should be 1440 × 1280 pixels. If the display is 1080 × 1920, the wallpaper should be 2160 × 1920 pixels.
You copy the generated image file in the phone memory and set as wallpaper.
How many power saving black wallpaper
The practical test showed an increase in smartphone operating time by 20-30% when continuously on the home screen.