void setup() {
  size(200, 200);
  background(255);
  noStroke();
}

void loop() {

  // this is not the black square
  // we just paint a semitransparent white square to fade
  fill(255,50);
  rect(0,0,200,200);

  // now this is the black square
  int deltax=pmouseX-mouseX;
  int deltay=pmouseY-mouseY;
  fill(0,100);
  quad(25+deltax,25+deltax,175+deltay,25+deltay,175+deltax,175+deltax,25+deltay,175+deltay);

}