Something like that:
http://jsfiddle.net/Volter9/cMWv6/
ctx.clearRect(0,0,400,400);
ctx.beginPath();
ctx.arc(100,75,50,0,2*Math.PI);
ctx.strokeStyle="#fff";
ctx.fillStyle="#fff";
ctx.shadowBlur=SB;
Your key problem was you forgot to plug in shadowBlur your variable and you forgot to clear the canvas.
Good luck!