XPLMDrawTranslucentDarkBox
From X-Plane SDK
XPLMDrawTranslucentDarkBox
XPLM_API void XPLMDrawTranslucentDarkBox(
int inLeft,
int inTop,
int inRight,
int inBottom);
This routine draws a translucent dark box, partially obscuring parts of the screen but making text easy to read. This is the same graphics primitive used by X-Plane to show text files and ATC info.
X-Plane SDK