Customize beamer block style

I am using Montpellier theme with beamer, however the block style have no default background etc. I tried a few ways to make it look like the style of Boadilla, but I can't reproduce it good enough.

Being new to beamer, the search led me to customize it with setbeamercolor, but I am lost on how exactly specify color the way Boadilla did. Can you help?

\documentclass[c,compress]{beamer}
\usetheme{Montpellier}
\usecolortheme{beaver}
\useinnertheme[shadow]{rounded}

%\setbeamercolor{block title}{use=structure,fg=white,bg=blue!75!black}

\title{My title}

\begin{document}

\section{First Section}

\begin{frame}
    \frametitle{A Test}
    \begin{block}{definition}
        a test for block
    \end{block}
\end{frame}

\end{document}