\documentclass[frenchb]{article}
\usepackage{babel,hyperref}
%% the idea with futurelet comes from Torsten Hilbrich (April 2001 in dctt)
\makeatletter
\declare@shorthand{french}{:}{\texorpdfstring{\futurelet\token\checkit}{
        \string:}}
\def\checkit{\ifx\token\@sptoken\ifhmode\ifdim\lastskip>\z@
  \unskip\penalty\@M\ \else\FDP@space\fi\fi\fi\string:}
\makeatother
\begin{document}
\section{bla: bl\"a}
bla: bla und bla:bla
\end{document}