For bullseye pistol competition.  Should I get a white one or a black one?  What's the difference?

Get the white one. Either will cause you to concentrate with your aiming eye. However, the dark one will cause more of a strain between the eyes. The lighter/white one will be in balance with the aiming eye.

Thanks.  Anyone know of an on-line store with one in stock?

This, you still want to see from the other eye.

Thanks, guys.

I found one and ordered from CHAMPIONSHOOTERS.COM.

try the following
Champions Choice
Creedmoor Sports

I had bought a black one and it works fine.  The white one will be a benefit if you shoot under a covered firing line or indoors, since it reflects more light.

I used to clip it to my glasses and occlude my vision totally.  I thought this was ok, and then this year another shooter told me to try putting it out on my ball cap.  It took a little getting used to, but actually helped a lot with my sight picture.  The idea is to block your eye from seeing the target, but still let as much light in to your non-dominant eye as the dominant eye.  The "natural" light to both eyes helps keep your dominant eye from straining to get proper sight alignment and sight picture.

That's why a lot of people like the white blinders, more light reflection.