\setupbackend[level=0]
\setuppagenumber[state=stop]
  
\starttext
\ctxlua{metapost.showlog = true}%

\startuniqueMPgraphic{blacktowhite}
    linear_shade(OverlayBox,0,white,black);
\stopuniqueMPgraphic
\defineoverlay[blacktowhite][\uniqueMPgraphic{blacktowhite}]

\framed[width=100mm,height=70mm,background=blacktowhite,frame=off,offset=overlay,empty=yes]{}
\stoptext
