Using \only on a block - beamer

I don't see your problem. I built this MWE:

\documentclass{beamer}
\usetheme{CambridgeUS}

\begin{document}

\begin{frame}{Title of this frame}
% first block appears at first
\only<1>{
    \begin{block}{First block}
    Hello
    \end{block}
}
% first block disappears and the second appears
\only<2>{
    \begin{block}{Second block}
    Hello again
    \end{block}
}

\end{frame}

\end{document}

getting these two frames:

As you can see, the frame number is always the same and the second block appears after the first one, that is not more visible.

Edit

After you posted your MWE I got your problem. Maybe my solution will not completely satisfy you, but it works.

First: the option fragile is needed anyway. Secondly, since I noticed that the code occupies more or less the whole frame (even setting the font size smaller), I decided in my solution to put it outside the itemize environment.

This is the new MWE:

\documentclass{beamer}
\usetheme{CambridgeUS}

\usepackage[french]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{moreverb}
\usepackage{listings}
\lstset{basicstyle={\footnotesize}}
\begin{document}

\begin{frame}[fragile]{Creation des outils principaux}
  % - Le titre doit résumer le transparent dans un langage compréhensible
  %   par tous ceux qui ne suivront rien de ce qu'il y a sur ce transparent.
\begin{itemize}
\item<1-> Recuperation des donnees entrées dans le terminal
\end{itemize}
\begin{exampleblock}{Opt Parse}<2>
\begin{lstlisting}[firstnumber=0]
import optparse
def parser():
    parser = optparse.OptionParser(        description = "Dessinateur de fonctions")
    parser.add_option('-o', type=str, action = "store",
                        default="out.ppm", dest="filename",
                        help='filename. Default : out.ppm')
    return parser.parse_args()
if __name__ == '__main__':
    (opts, args) = parser()
\end{lstlisting}
\end{exampleblock}

\end{frame}
\end{document}

The result is shown in the following two frames:

Notice that the key point is having inserted <1-> to the item and <2> for the exampleblock. In this way the sentence of the item is still visible when the exampleblock appears.

A variant of Claudio's answer can be obtained by modifying the action specifications of the block environment, as explained in the Beamer User Guide for version 3.50 in Section 9.6.3.:

\documentclass{beamer}
\usetheme{CambridgeUS}

\begin{document}

\begin{frame}{Title of this frame}
% first block appears at first
\begin{block}<only@1>{First block} % <-- note the "only@" here...
    Hello
\end{block}
% first block disappears and the second appears
\begin{block}<only@2>{Second block} % <-- and here
    Hello again
\end{block}

\end{frame}

\end{document}