hyperref: wrapper around \href is fails to compile when placed in the argument of another command

Adapting from https://tex.stackexchange.com/a/448977/383565, I constructed my own version of \blink. But, it doesn’t work in captions, unlike the base \href:

\documentclass[a4paper, 11pt]{book}
\usepackage{tikz, soul, hyperref}
% External hyperlink special formatting
  % External link symbol
  % source: https://tex.stackexchange.com/a/294990/383565
  \newcommand{\ExternalLink}{%
      \tikz[x=1.2ex, y=1.2ex, baseline=-0.05ex]{% 
          \begin{scope}[x=1ex, y=1ex]
              \clip (-0.1,-0.1) 
                  --++ (-0, 1.2) 
                  --++ (0.6, 0) 
                  --++ (0, -0.6) 
                  --++ (0.6, 0) 
                  --++ (0, -1);
              \path[draw, 
                  line width = 0.5, 
                  rounded corners=0.5] 
                  (0,0) rectangle (1,1);
          \end{scope}
          \path[draw, line width = 0.5] (0.5, 0.5) 
              -- (1, 1);
          \path[draw, line width = 0.5] (0.6, 1) 
              -- (1, 1) -- (1, 0.6);
          }%
      }
  % xunderline
  % source: https://tex.stackexchange.com/a/67008/383565
  \makeatletter
  \ExplSyntaxOn
  \cs_new:Npn \white_text:n #1
    {
      \fp_set:Nn \l_tmpa_fp {#1 * .01}
      \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{\fp_to_decimal:N \l_tmpa_fp em}}
      \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{-\fp_to_decimal:N \l_tmpa_fp em}}
    }
  \NewDocumentCommand{\whiten}{ m }
      {
        \int_step_function:nnnN {1}{1}{#1} \white_text:n
      }
  \ExplSyntaxOff

  \NewDocumentCommand{ \xunderline }{ D<>{5} O{0.2ex} O{0.1ex} +m } {%
  \begingroup
  \setul{#2}{#3}%
  \def\SOUL@uleverysyllable{%
    \setbox0=\hbox{\the\SOUL@syllable}%
    \ifdim\dp0>\z@
        \SOUL@ulunderline{\phantom{\the\SOUL@syllable}}%
        \whiten{#1}%
        \llap{%
          \the\SOUL@syllable
          \SOUL@setkern\SOUL@charkern
        }%
    \else
        \SOUL@ulunderline{%
          \the\SOUL@syllable
          \SOUL@setkern\SOUL@charkern
        }%
    \fi}%
      \ul{#4}%
  \endgroup
  }
  \makeatother
  % ======================================== %
  %* ===== MOST IMPORTANT PART OF MWE ===== *%
  % ======================================== %
  % href with underline and an external link symbol
  \NewDocumentCommand{\blink}{ O{} }{%
    %% We want the #-sign only to be a letter (catcode 11) temporarily, so begin the scope group
    \bgroup%
    % Below, # is made a letter until \egroup is called
    \catcode`\#=11\relax%
    % Trigger the URL-reader
    \makeblinkurl%
  }
  \NewDocumentCommand{\makeblinkurl}{ m m }{%
    % This will read the link and link description
    % after the # is made a letter by the \blink macro
    \href{#1}{\xunderline{#2}\:\ExternalLink}%
    % We want the #-sign only to be a letter up til here, so we group it.
    % the broup will start in the below \blink macro,
    % and we wnd it here:
    \egroup%
  }
\begin{document}
This compiles: \blink{https://docs.python.org/3.7/library/stdtypes.html#string-methods}{string methods}

% \begin{figure}[htbp]
%     \caption{This doesn't compile \blink{https://docs.python.


org/3.7/library/stdtypes.html#string-methods}{string methods}.}
% \end{figure}

% \section{This also doesn't compile: \blink{https://docs.python.org/3.7/library/stdtypes.html#string-methods}{string methods}}
\end{document}

When we uncomment either the figure float or the section header, the following errors arise:

./tst.tex:89: Illegal parameter number in definition of \GetTitleStringResult.
<to be read again> 
                   s
l.89 ...pes.html#string-methods}{string methods}.}
                                                  
./tst.tex:89: Illegal parameter number in definition of \GTS@GlobalString.
<to be read again> 
                   s
l.89 ...pes.html#string-methods}{string methods}.}
                                                  
./tst.tex:89: Illegal parameter number in definition of \reserved@a.
<to be read again> 
                   s
l.89 ...pes.html#string-methods}{string methods}.}
                                                  
./tst.tex:89: Illegal parameter number in definition of \Hy@tempa.
<to be read again> 
                   s
l.89 ...pes.html#string-methods}{string methods}.}
                                                  
./tst.tex:89: Illegal parameter number in definition of \Hy@tempa.
<to be read again> 
                   s
l.89 ...pes.html#string-methods}{string methods}.}
                                                  
./tst.tex:89: Illegal parameter number in definition of \Hy@tempa.
<to be read again> 
                   s
l.89 ...pes.html#string-methods}{string methods}.}

Previously, I had already managed to make such a command \blink work by copying the source definition of \href with slight modifications. But, I rather not touch package internals at all. So, I would appreciate if someone tell me how to get such a wrapper \blink around \href to work correctly.