How to Darken an Irregularly Shaped Transparent CALayer on the iPhone?

Question

I'm putting an image into a CALayer that could be irregularly transparent:

 theCardLayer.front = [CALayer layer];
 theCardLayer.front.contents = (id)[cardDrawing CGImage];

In other words, it might be a square filling the layer or it might be an octagon that leaves the corners see-through.

I want to sometimes darken this layer, but without darkening the see-through bits. Any suggestions for how to do so in a programmatic way?