Re: Silicone glass pre foam.
I actually just finished a build and they have black foam currently. It's Great Stuff Pond and Stone. Wasn't able to find it at Home Depot but it was relatively cheap online - $9 per can. I filled the back and the sides with the black foam and stacked the normal Great Stuff product in the center to build structure. All of it was covered with black silicone. However, any of the foam visible from the outside appears black.