\setupbodyfont[20pt]

\showframe

\startuseMPgraphic{fuzzy}

path p ; p := (0,0) -- (0,OverlayHeight) -- (OverlayWidth,OverlayHeight) --
(OverlayWidth,0) -- cycle ;

pickup pencircle scaled .5pt ;

for i=1 upto 20 :
 draw p enlarged -5pt randomized 15pt withcolor blue ;
endfor ;

\stopuseMPgraphic

\defineoverlay [Fuzzy] [\useMPgraphic{fuzzy}]

\startTEXpage


\framed [height=20cm,width=12cm,frame=on,background=Fuzzy,align={middle,lohi}]
{Test}

\stopTEXpage