Blank Child Theme is nothing but a Blank Theme which is used for modification purpose.
A child theme is a theme that has all the functionality and styling of another theme called the parent theme, which in our case, is WPLMS. Child themes are the recommended way of modifying the code of an existing theme because a child theme preserves all custom code changes and modifications even after theme update. If you modify code directly from a parent theme, then update the parent theme, your changes will be lost. Always use a child theme if you are modifying the core code. Child themes can also be used for a number of things, such as custom CSS applications, template file modifications, and custom PHP functions and/or hooks. There are a couple of methods to modify a child theme. Child themes don’t guarantee that an update of custom code on the parent theme will not require further maintenance. This is especially true if you copy files from the parent theme to your child theme.
How to Activate Blank Child Theme
refer: https://screencast-o-matic.com/watch/cYnuiiw9mn
Sometimes You have to set the display location of the menus again after activating Blank Child Theme. After activating the Child Theme the site looks as it was before. Nothing will be changed. But now you can customize your site using this theme without any fear that you will lose your custom code.