\documentclass[12pt]{article}
\usepackage{bm}
\usepackage{upgreek}
\makeatletter
\newcommand{\bfgreek}[1]{\bm{\@nameuse{up#1}}}
\makeatother
\parindent=0pt
\begin{document}
The default upright $\upmu$. Written with \verb|$\upmu$| in mathmode.

\verb|$\bfgreek{mu}$| will allow you to type $\bfgreek{mu}$ to obtain an upright boldface $\mu$.
\end{document}
