How to visually “earmark” certain slides?

Question

In a Beamer presentation, instead of using appendix slides I find it sometimes useful to keep certain slides as part of the flow, but throw in a visual "earmark".

I wonder how to create a small command that if contained in a slide would add a semi-transparent triangle in the corner, like below:

Answer 1

I provide \earmark in the manner requested.

To elaborate, I use the atbegshi package to allow things to be added at page shipout. I create the \atxy{<x>}{<y>}{<content>} macro for placing these shipout items in the foreground at the specified (x,y) location. Finally, I define \earmark to place a transparent triangle at (x,y) = (0pt, \paperheight) using \atxy.

I apologize that my brute force tikz transparent triangle is a result of my poor tikz knowledge.

\documentclass{beamer}
\usepackage{atbegshi}
\usepackage{lipsum,tikz}
\def\PageTopMargin{1in}
\def\PageLeftMargin{1in}
\newcommand\atxy[3]{%
 \AtBeginShipoutNext{\AtBeginShipoutAddToBoxForeground{%
  \smash{\hspace*{\dimexpr-\PageLeftMargin-\hoffset+#1\relax}%
  \raisebox{\dimexpr\PageTopMargin+\voffset-#2\relax}{#3}}}}}
\def\earmark{%
  \atxy{0pt}{\paperheight}{\begin{tikzpicture}
  \node[outer sep=0.46in] (earmark) {};
  \draw[blue!40, fill, opacity=0.25] (earmark.south west) -- (earmark.south east) -- 
    (earmark.north west)-- (earmark.south west);
  \end{tikzpicture}}
}
\begin{document}
\begin{frame}{title of the slide}
\earmark
\lipsum[3]
blah blah\par
blah blah\par
And here we test opacity
\end{frame}
\begin{frame}{next slide}
There is no earmark here
\end{frame}
\end{document}