Unable to remove action from parent theme via child theme

The child theme’s functions.php is loaded before the parent theme’s functions.php:

Unlike templates and patterns, the functions.php file of a child theme does not override the functions.php file in the parent theme. In fact, they are both loaded, with the child being loaded immediately before the parent.

Source:
https://developer.wordpress.org/themes/advanced-topics/child-themes/#using-functions-php

This means that when the child theme’s remove_action() is called, there is no action to remove. To circumvent this, try this in the child theme’s functions.php (untested):

add_action( 'wp_footer', static function () {
    remove_action( 'wp_footer', 'blankslate_footer', 10 );
}, 9 );

This should work, because it hooks into the same action with an earlier priority, and removes the hook (if it exists).