Add a column to the Reusable Blocks screen to show the posts the block is used in.

my_reusable_blocks_screen_add_column.php

<?php
/**
 * Add column to show all posts a reusable block is used in.
 */
function my_reusable_blocks_screen_add_column( $columns ) {
	$columns['posts_used_in'] = esc_html__( 'Used In', 'textdomain' );
	return $columns;
}
add_filter( 'manage_wp_block_posts_columns', 'my_reusable_blocks_screen_add_column' );

/**
 * Populate the 'posts_used_in' column of the Reusable Blocks screen.
 *
 * This code is adapted from https://wordpress.org/plugins/reusable-blocks-extended/
 */
function my_reusable_blocks_screen_populate_column( $column, $ID ) {
	global $post;
	switch( $column ) {
		case 'posts_used_in' :
			global $wpdb;
			$tag = '<!-- wp:block {"ref":' . $ID . '}';
			$search = '%' . $wpdb->esc_like($tag) . '%';
			$instances = $wpdb->get_results( "SELECT * FROM {$wpdb->prefix}posts WHERE post_content LIKE '$search' and post_type NOT IN ('revision', 'attachment', 'nav_menu_item')" );
			if ( ! empty( $instances ) ) {
				$post_links = array();
				foreach ( $instances as $instance ) {
					$post_links[] = '<a href="' . get_edit_post_link( $instance->ID ) . '">' . $instance->post_title . '</a>';
				}
				echo implode( ', ', $post_links );
			} else {
				esc_html_e( '&#8212;', 'textdomain' );
			}
			break;

		default :
			break;
	}
}
add_action( 'manage_wp_block_posts_custom_column' , 'my_reusable_blocks_screen_populate_column', 10, 2 );