def _gradient_black_lines(
            self, text, start, step=1,
            fore=None, back=None, style=None, reverse=False,
            movefactor=2, rgb_mode=False):
        """ Yield colorized characters,
            within the 24-length black gradient,
            treating each line separately.
        """
        if not movefactor:
            def factor(i):
                return start
        else:
            # Increase the start for each line.
            def factor(i):
                return start + (i * movefactor)
        return '\n'.join((
            self._gradient_black_line(
                line,
                start=factor(i),
                step=step,
                fore=fore,
                back=back,
                style=style,
                reverse=reverse,
                rgb_mode=rgb_mode,
            )
            for i, line in enumerate(text.splitlines())
        ))
Yield colorized characters,
            within the 24-length black gradient,
            treating each line separately.