\setupcolors[state=start]

\usemodule[tikz]
\usetikzlibrary{fadings}

\starttext

\tikzfading[name=fadeout,
  innercolor=transparent!0,
  outercolor=transparent!100]

%Now we use the fading in another picture:
\starttikzpicture
  %Background
  \fill[black!20](-1.2,-1.2)rectangle(1.2,1.2);
  \path[pattern=checkerboard,patterncolor=black!30]
        (-1.2,-1.2)rectangle(1.2,1.2);
  \fill[blue,pathfading=fadeout](-1,-1)rectangle(1,1);
\stoptikzpicture

\stoptext