Trouble with clipping mask process

Hello,

I am trying to find the best process to create a mask like the following.

I'm starting with a black image.

Creating splotches behind it.

Now here's the part I'm having trouble with. I want to basically make a circle of splotches. I originally made a white rectangle and then clipped it so the circle was transparent. This gives me my desired result except the background is white rather than transparent. I hope this makes any sense.

Thanks!