init.php

function genesis_sample_blocks_body_classes( $classes ) {

	if ( ! is_singular() || ! function_exists( 'has_blocks' ) || ! function_exists( 'parse_blocks') ) {
		return $classes;
	}

	if ( ! has_blocks() ) {
		$classes[] = 'has-no-blocks';
		return $classes;
	}

	$post_object = get_post( get_the_ID() );
	$blocks      = (array) parse_blocks( $post_object->post_content );

	// foreach( $blocks as $block ) {
	//	echo 'Name: ' . $block['blockName'] . '<br/>';
	//	echo 'Alignment: ' . $block['attrs']['align'] . '<br/>';
	//	echo '<br/>';
	// }

	//* Output the "first" classes
	if ( isset( $blocks[0]['blockName'] ) ) {
		$classes[] = 'first-block-' . str_replace( '/', '-', $blocks[0]['blockName'] );
	}

	if ( isset( $blocks[0]['attrs']['align'] ) ) {
		$classes[] = 'first-block-align-' . $blocks[0]['attrs']['align'];
	}

	//* Get the "last" classes
	foreach ( $blocks as $block ) {
		$lastclassname = null;
		$lastclassalign = null;

		if ( $block['blockName'] )
			$lastclassname = 'last-block-' . str_replace( '/', '-', $block['blockName'] );
		
		if ( $block['attrs']['align'] )
			$lastclassalign = 'last-block-align' . str_replace( '/', '-', $block['attrs']['align'] );
	}

	//* Output the "last" classes
	if ( isset( $lastclassname ) )
		$classes[] = $lastclassname;

	if ( isset( $lastclassalign ) )
		$classes[] = $lastclassalign;

	return $classes;

}