This article is from the Stand-up Comedy FAQ, by Steven Rosenthal and Steve Silberberg (firstname.lastname@example.org) with numerous contributions by others.
A. Working blue refers to using off-color, not suitable for TV material.
While four letter words certainly fall into this category, working blue is
not limited to expletives. For instance, describing various secretions in
great detail can also constitute working blue, despite the fact that you
may have used acceptable medical terminology only.