With flat shadow I am referring to a shadow of a single color without a gradient. Ideally, I would like to have an opaque drop shadow in the style of the tikz.shadows library. It can be used as follows to add a flat shadow to a node:

 \node[draw,drop shadow={opacity=.25, shadow xshift=0.07, shadow yshift=-0.07, color=green}] at (0,0) {node with green shadow};


This is my MWE beamer document containing a single box with a fading shadow:

\documentclass{beamer}

\begin{document}
\begin{frame}{Example frame}
\end{block}
\end{frame}
\end{document}


The definition of the box shadows is most certainly contained in beamerbasedboxes.sty of the beamer package. Unfortunately, I do not understand the code since it mostly uses low level pgf commands instead of tikz commands. So far, I have tried to modify several commands in this file, but without success. Or maybe I have simply overlooked an option of the blocks template?

While someone provides a only-beamer-solution, you could consider to use tcolorbox and its easily customizable boxes.

\documentclass{beamer}

\usepackage[most]{tcolorbox}

\begin{document}
\begin{frame}{Example frame}
\end{block}

I, box, have a flat shadow.
\end{tcolorbox}

\end{frame}
\end{document}


2nd version:

Although I'm don't know what I did, after some tests I've got

from:

\documentclass{beamer}

\makeatletter
color(0bp)=(black!20!bg); color(4bp)=(black!20!bg)}
color(0bp)=(black!20!bg); color(4bp)=(black!20!bg); color(6bp)=(black!20!bg)}
\makeatother

\begin{document}
\begin{frame}{Example frame}

• @ Ignasi: Ok! I have found it that the effect can be mostly eliminated by commenting out the big block \ifbmb@shadow% { \color{black!50!bg} ....... } in the file beamerbaseboxes.sty. However, I have no idea why. The code is pretty heavy. – Hendrik Nov 25 '15 at 14:57