(For testing purposes) WP API endpoint for parsing a post for blocks.

posts-blocks-endpoint.php

<?php
/**
 * Plugin Name: Blocks endpoint for Posts
 * Description: —
 */

add_action( 'rest_api_init', 'mcsf_register_post_blocks_endpoint' );
function mcsf_register_post_blocks_endpoint() {
	register_rest_route(
		'wp/v2', '/posts/(?P<id>[\d]+)/blocks', array(
			'args' => array(
				'id' => array(
					'description' => 'Unique identifier',
					'type' => 'integer',
				),
			),
			array(
				'methods' => WP_REST_Server::READABLE,
				'callback' => 'mcsf_get_post_blocks',
			),
		)
	);
}

function mcsf_get_post_blocks( $request ) {
	$post = get_post( $request['id'] );
	if ( is_wp_error( $post ) ) {
		return $post;
	}

	$data     = gutenberg_parse_blocks( $post->post_content );
	$response = rest_ensure_response( $data );
	return $response;
}