"Black marker" effect in the text (sanitization used to hide passwords in code example)
There also is the Steven B. Segletes's censor
package (documentation).
You can use \censor{blurg}
to censor short pieces of text.
Long paragraphs can be blacked out with \blackout{foo\par bar}
.
Censoring can be turned off via \StopCensoring
.
\documentclass{article}
\usepackage{censor}
\begin{document}
When shall we \censor{three meet again}, in thunder, lightning or in rain?
Hi \blackout{Hello
Goodbye} Bye
\StopCensoring
When shall we \censor{three meet again}, in thunder, lightning or in rain?
\end{document}
Depending on whether you want to mark only short strings of text you could do something like this:
\documentclass{article}
\usepackage{calc}
\newif\ifmarksecrets % declare new switch
\marksecretstrue %comment or use \marksecretsfalse to display normal text
\newcommand{\secret}[1]{
\ifmarksecrets % Show black rule
\rule{\widthof{#1}}{\heightof{#1}}
\else % Just display text
#1
\fi
}
\begin{document}
Foo foo foo \secret{VERY SECRET BAR} foo foo foo.
\end{document}
For larger boxes, TikZ can be used to draw anything anywhere on the page, but I'm in a bit of a hurry so I can't provide an example now.