I want to replace my present pretty picture wallpaper which obscures stuff to a
blank wallpaper. I found a plain black wallpaper on play store and clicked on
‘install’ … but nothing got installed [i.e.] no change in cellphone wallpaper. please advise.
You have to go into your settings and select the app that you downloaded, then choose the wallpaper you want. I'm not sure what device you own but long press on your screen and then choose wallpaper, then either choose your app or "other wallpaper apps" (or something similar), then choose your wallpaper.
You installed the black wallpaper to your device, but didn't tell your device to USE it as the wallpaper. Now that it's installed, long-press a blank part of your homescreen, select "wallpapers" and then choose your desired black wallpaper (which should now appear on the menu, probably toward the end since it was the last one you installed). Once you do that, you'll have a nice, black wallpaper.