\documentclass[glossy]{beamer}
\useoutertheme{wuerzburg}
\useinnertheme[realshadow,corners=2pt,padding=2pt]{chamfered}
\usecolortheme{shark}
\usepackage{tikz}
\newcommand<>{\hover}[1]{\uncover#2{%
\begin{tikzpicture}[remember picture,overlay]%
\draw[fill,opacity=0.4] (current page.south west)
rectangle (current page.north east);
\node at (current page.center) {#1};
\end{tikzpicture}}
}
\title{Your Title}
\author{Your Name}
\institute{Your Institute}
\date{\today}
\begin{document}
\begin{frame}
\maketitle
\end{frame}
\begin{frame}
\frametitle{This is a slide}
\begin{itemize}
\item These are more points
\item These are more points
\item These are more points
\item These are more points
\item These are more points
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{A frame}
Some text
\hover<2>{
\begin{minipage}{0.8\linewidth}
\begin{block}{A block hovering above the slide}
I am visible on slide two.
\end{block}
\end{minipage}
}
\end{frame}
\end{document}
