I have come across a similar need before. You will need to know the layer that the BlockUI is on. I will assume that it has a z-index of 10. What you are going to want to do here is set an invisible layer beneath it and then attach a click event there which will tie into the close event already on your blockui element.
.clr{
position:fixed;
width:100%;
height:100%;
z-index:5;
top:0;
left:0;
bottom:0;
right:0;
}