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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def mark_bl(self) -> int: """ 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. """

        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