Consider
\documentclass{article}
\usepackage{titlesec}
\def\bl#1 #2 {#1\textsuperscript{#2}\,}
\titleformat{\section}[runin]{}{}{0pt}{}[. ]
%\titleformat{\section}[runin]{}{}{0pt}{\bl}[. ]
\begin{document}
\section{3 a 21}
main text.
\end{document}
The intention of the line commented out was to produce the æquivalent of \section{\bl 3 a 21}
without having to type \bl for each section; but it produced an error. Can something be done to achieve the intended result?