get_subdirectory_reserved_names()

function get_subdirectory_reserved_names() {
	$names = array(
		'page',
		'comments',
		'blog',
		'files',
		'feed',
		'gc-admin',
		'gc-content',
		'gc-includes',
		'gc-json',
		'embed',
	);

	/**
	 * Filters reserved site names on a sub-directory Multisite installation.
	 *
	 * @since 3.0.0
	 * @since 4.4.0 'gc-admin', 'gc-content', 'gc-includes', 'gc-json', and 'embed' were added
	 *              to the reserved names list.
	 *
	 * @param string[] $subdirectory_reserved_names Array of reserved names.
	 */
	return apply_filters( 'subdirectory_reserved_names', $names );
}

/**
 * Modify reserved names for sites.
 *
 * @param $names array List of reserved names for sites.
 *
 * @return array Updated list of reserved names for sites.
 */
function change_subdirectory_reserved_names( $names ) {
	$position = array_search( 'blog', $names );

	if( $position ) {
		unset( $names[$position] );
	}

	return $names;
}
add_filter( 'subdirectory_reserved_names', 'change_subdirectory_reserved_names' );