How to put the block title at the center in beamer?

Actually - I found a much easier way of centering the title within a block. I have to say, I'm not sure if this is sort of hacky, but it works.

\begin{block}{\centering 1. Introduction}
...
...
\end{block}

I'm imagining it would work similarly for your varblock environment.

\begin{varblock}[35cm]{\centering \textbf{1. Introduction}} 
\justifying Space for discussion. 
\end{varblock}

Change your definition of varblock to

\newenvironment<>{varblock}[2][\textwidth]{% 
  \setlength{\textwidth}{#1} 
  \begin{actionenv}#3% 
  \def\insertblocktitle{\centering#2\par}% 
  \par% 
  \usebeamertemplate{block begin}} 
{\par% 
  \usebeamertemplate{block end}% 
  \end{actionenv}}