Wordpress check if block is used first

block-helpers.php

<?php

/**
 * Block helpers
 * add special classes when certain blocks appear, put this in your functions.php file or include it somewhere
 */

// add block classes in body and post class
function blocks_body_class( $classes ) {
	global $post;

	// check if a block is first
	if ( has_blocks( $post->post_content ) ) {
	    $blocks = parse_blocks( $post->post_content );

	    if ( $blocks[0]['blockName'] === 'core/image' ) {
	    	$classes[] = 'image-first';
	    }

	}
	
	// check if a block is used anywhere at all
	if ( has_block( 'custom/hero' ) ) {
		$classes[] = 'hero';
	}

	return $classes;
}

add_filter( 'post_class', 'blocks_body_class' );
add_filter( 'body_class', 'blocks_body_class' );