$blocks = parse_blocks( get_the_content() );
  $block_ids = array();

  function get_block_id( $block ) {
    if ( $block['blockName'] !== 'core/heading' ) return;

    $block_html = $block['innerHTML'];
    $id_start = strpos( $block_html, 'id="' ) + 4;

    if ( $id_start === false ) return;

    $id_end = strpos( $block_html, '"', $id_start );
    $block_id = substr( $block_html, $id_start, $id_end - $id_start );

    return $block_id;
  }

  foreach( $blocks as $block ) {
    $block_id = get_block_id( $block );
    if ( $block_id ) array_push ( $block_ids, $block_id );
  }