code: def mark_bl(self) -> int:
          
          counter = 0
          stringy_lines = find_stringy_lines(self.node, self.first_line_no)
          for relative_line_number, line in enumerate(self.lines):
              if relative_line_number not in stringy_lines and line.strip() == '':
                  counter += 1
                  self.line_markers[relative_line_number] = LineType.blank_line
  
          return counter
docs: Mark unprocessed lines that have no content and no string nodes
          covering them as blank line BL.
  
          Returns:
              Number of blank lines found with no stringy parent node.