The inheritance of fonts in blocks seems a bit inconsistent, maybe this can be unified...
\documentclass{beamer}
\setbeamerfont{block body}{series=\itshape}
\begin{document}
\begin{frame}
\begin{block}{title}
content...
\end{block}
\begin{exampleblock}{title}
content...
\end{exampleblock}
\begin{alertblock}{title}
content...
\end{alertblock}
\end{frame}
\end{document}

The inheritance of fonts in blocks seems a bit inconsistent, maybe this can be unified...