Blocks in Beamer

Your MWE is using the default beamer theme, which doesn't define the blocks to have the colors you want. Try using a different theme, as below. Also, to get blue/green/red use block/exampleblock/alertblock, respectively.

\documentclass[pdf]{beamer}
\usetheme{Copenhagen}
\begin{document}
\begin{frame}
\frametitle{Frame title}
\begin{block}{Observation 1}
Simmons Hall is composed of metal and concrete.
\end{block}
\begin{exampleblock}{Observation 2}
Simmons Dormitory is composed of brick.
\end{exampleblock}
\begin{alertblock}{Conclusion}
Simmons Hall $\not=$ Simmons Dormitory.
\end{alertblock}
\end{frame}
\end{document}

You can also customize how things look by using \useinnertheme{} and \usecolortheme{}. See the beamer documentation for more info.

This has already been answered but I should like to add that you can still use whichever theme you want with invisible blocks, and then set the colors to make blocks apparent. In the preamble you could include those parameters for instance,

\setbeamercolor{block body alerted}{bg=alerted text.fg!10}
\setbeamercolor{block title alerted}{bg=alerted text.fg!20}
\setbeamercolor{block body}{bg=structure!10}
\setbeamercolor{block title}{bg=structure!20}
\setbeamercolor{block body example}{bg=green!10}
\setbeamercolor{block title example}{bg=green!20}

With these settings you would get green blocks for examples, blocks of the color of the alert text for alert boxes, and the color of the structure for the default boxes, so it follows somehow the theme you're using.

Ah, and you might want to add this line to round the edges of your blocks and cast a shadow. Some templates with invisible block contours might not have included this.

\setbeamertemplate{blocks}[rounded][shadow]

You need to use a theme to have blocks appearing this way. Here is an example with "Boadilla"

\documentclass[10pt,a4paper]{beamer}
\usepackage[utf8]{inputenc}

\usetheme[secheader]{Boadilla}

\begin{document}

\begin{frame}{Title Frame}
    Introduction sentece
    \begin{block}{Title Block 1}
    {
        \begin{itemize}
        \item item 1
        \item item 2
        \end{itemize}
    }
    \end{block}
    \begin{exampleblock}{Title Block 2}
    {
        Text.
    }
    \end{exampleblock}
    \begin{alertblock}{Title Block 3}
    {
        You
    }
    \end{alertblock}    
\end{frame}

\end{document}

giving the result :

You need to use block, exampleblock and alertblock.