Frank Goossens (Futta) function to extract blocks from WP Gutenberg (Proof of Concept) as https://blog.futtta.be/2018/01/25/how-to-extract-blocks-from-gutenberg/

futta-gutenprint.php

// proof-of-concept to extract all WordPress Gutenberg's blocks as array
// author: Frank Goossens (Futta)
// source: https://blog.futtta.be/2018/01/25/how-to-extract-blocks-from-gutenberg/

add_action('the_content','gutenprint',10,1);
function gutenprint($html) {
    // check if we need to and can load the Gutenberg PEG parser
    if ( !class_exists("Gutenberg_PEG_Parser") && file_exists(WP_PLUGIN_DIR."/gutenberg/lib/load.php") ) {
        include_once(WP_PLUGIN_DIR."/gutenberg/lib/load.php");
    }
 
    if ( class_exists("Gutenberg_PEG_Parser") && is_single() ) {
      // do the actual parsing
      $parser = new Gutenberg_PEG_Parser;
      $result = $parser->parse(  _gutenberg_utf8_split( $html ) );
       
      // we need to see the HTML, not have it rendered, so applying htmlentities
      array_walk_recursive($result,
        function (&$result) { $result = htmlentities($result); }
        );
 
      // and dump the array to the screen
      echo "<h1>Gutenprinter reads:</h1><pre>";
      var_dump($result);
      echo "</pre>";
    } else {
      echo "Not able to load Gutenberg parser, are you sure you have Gutenberg installed?";
    }
   
    // remove filter to avoid double output
    remove_filter('the_content','gutenprint');
   
    // and return the HTML
    return $html;
}

here is a solution that works in wordpress 5

add_action('the_content','gutenprint',10,1);
function gutenprint($html) {
// check if we need to and can load the WP_Block_Parser
if ( !class_exists("WP_Block_Parser") && file_exists(ABSPATH . WPINC . '/class-wp-block-parser.php') ) {
    include_once( ABSPATH . WPINC . '/class-wp-block-parser.php' );
}

if ( class_exists("WP_Block_Parser") && is_single() ) {
    // do the actual parsing
    $parser = new WP_Block_Parser;
    $result = $parser->parse( $html );

    // we need to see the HTML, not have it rendered, so applying htmlentities
    array_walk_recursive($result,
        function (&$result) { $result = htmlentities($result); }
    );

    // and dump the array to the screen
    echo "<h1>Gutenprinter reads:</h1><pre>";
    var_dump($result);
    echo "</pre>";
} else {
    echo "Not able to load Gutenberg parser, are you sure you have Gutenberg installed?";
}

// remove filter to avoid double output
remove_filter('the_content','gutenprint');

// and return the HTML
return $html;
}