On Android 7.0 and above, Pitch Black Wallpaper uses the new APIs to set both the home screen and lock screen wallpaper to pitch black.
If you would like to choose whether to set the wallpaper on the Home screen, on the Lock screen, or on both screens, try Pitch Black Wallpaper Pro:
On older Android devices, as a general rule, the Android wallpaper that is displayed on the Home screen is also displayed on the Lock screen.
However, some device manufacturers have decided to use a separate image for the Lock screen, and there is no standard programmable way to change this image from an app.
If after running Pitch Black Wallpaper, your lock screen is still not black (pitch black), you have two options:
Option 1: Use Pitch Black Wallpaper Pro
The Pro version of Pitch Black Wallpaper lets you download a pitch black wallpaper image specifically tailored for your device. (It also lets you set and download wallpapers made with hundreds of Pitch Black alternatives, shades of black, dark and custom color wallpapers.)
Option 2: Use a generic black image
If the image does not download when tapping the button, long tap it instead and select Save Link. Don't worry, the image size is less than half a KB!
Setting the downloaded image as the Lock screen wallpaper
You can use the built-in method offered by Android to set/change the wallpaper (from the Home screen, from the Settings app, from the Photos app, and so on). Just pick the black wallpaper image you have just downloaded.
You can read this useful tutorial for more information on how to manually change the wallpaper on your Android smartphone or tablet.