how to specify opacity of a page item in script?

In my following script:

var myDoc = app.documents.item(0);

var myPage = app.activeWindow.activePage;

var myBounds = myPage.bounds;

var pageWidth = myBounds[3]-myBounds[1];

var pageHeight = myBounds[2]-myBounds[0];

var myRectangle = myPage.appliedMaster.rectangles.add({geometricBounds:[0, 0, pageHeight, pageWidth]});

myRectangle.fillColor = "Black";

myRectangle.strokeWeight = "0";

I want to change opacity of the myRectangle object.

Can anyone help me with the correct command to specify opacity in javascript?