\frametitle{Custom blocks}

  {
    \setbeamercolor{block title}{fg=blue,bg=green}
    \setbeamercolor{block body}{fg=black,bg=pink!50}
    \begin{block}{A custom block, with ugly colors}
      \begin{verbatim}
{
    \setbeamercolor{block title}{fg=blue,bg=green}
    \setbeamercolor{block body}{fg=black,bg=pink!50}

    \begin{block}{A custom block, with ugly colors}
        ...
    \end{block}
}
      \end{verbatim}
    \end{block}
  }