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.