beamerinnerthemeblockborder.sty

% sets the block environment to have borders
% this modifies the default definition in beamerinnerthemedefault.sty
% the effect is achieved using nested beamercolorboxes
% it may be possible to do this in a simpler manner using the 
% tcolorbox package

\defbeamertemplateparent{blocks}[bordered]{block begin,block end,%
  block alerted begin,block alerted end,%
  block example begin,block example end}{}

% Standard block
\defbeamertemplate*{block begin}{bordered}{%
  \par\vskip\medskipamount%
  % border box
  \begin{beamercolorbox}[colsep*=1.25ex]{block title}%
    % title box
    \vskip-1ex
    \begin{beamercolorbox}[colsep*=0.75ex,dp=0ex]{block title}%
      \usebeamerfont*{block title}\insertblocktitle\par%
    \end{beamercolorbox}%
    {\parskip0pt\par}%
    % body box
    \usebeamerfont{block body}%
    \begin{beamercolorbox}[colsep*=0.75ex]{block body}
      {\parskip0pt\par}%
    }
\defbeamertemplate*{block end}{bordered}{%
  \end{beamercolorbox}%
  \vskip-0.75ex%
  \end{beamercolorbox}%
  \vskip\smallskipamount%
}

% Alerted block
\defbeamertemplate*{block alerted begin}{bordered}{%
  \par\vskip\medskipamount%
  % border box
  \begin{beamercolorbox}[colsep*=1.25ex]{block title alerted}%
    % title box
    \vskip-1ex
    \begin{beamercolorbox}[colsep*=0.75ex,dp=0ex]{block title alerted}%
      \usebeamerfont*{block title alerted}\insertblocktitle\par%
    \end{beamercolorbox}%
    {\parskip0pt\par}%
    % body box
    \usebeamerfont{block body alerted}%
    \begin{beamercolorbox}[colsep*=0.75ex]{block body alerted}
      {\parskip0pt\par}%
    }
\defbeamertemplate*{block alerted end}{bordered}{%
  \end{beamercolorbox}%
  \vskip-0.75ex%
  \end{beamercolorbox}%
  \vskip\smallskipamount%
}

% Example block
\defbeamertemplate*{block example begin}{bordered}{%
  \par\vskip\medskipamount%
  % border box
  \begin{beamercolorbox}[colsep*=1.25ex]{block title example}%
    % title box
    \vskip-1ex
    \begin{beamercolorbox}[colsep*=0.75ex,dp=0ex]{block title example}%
      \usebeamerfont*{block title example}\insertblocktitle\par%
    \end{beamercolorbox}%
    {\parskip0pt\par}%
    % body box
    \usebeamerfont{block body example}%
    \begin{beamercolorbox}[colsep*=0.75ex]{block body example}
      {\parskip0pt\par}%
    }
\defbeamertemplate*{block example end}{bordered}{%
  \end{beamercolorbox}%
  \vskip-0.75ex%
  \end{beamercolorbox}%
  \vskip\smallskipamount%
}