it means the foam, when it is still liquid, has carbon added to it, so that when it dries in its pyramid shaped molds...the carbon is ~uniformly distributed thru it.
it is pretty hard to get the foam to be nonreflective any other way. they pyramidal shape is important, but you do not want it so heavily loaded (like a surface finish only) so that you do not have a big surface reflection.
I experimented a little once with iron filled paint. they make a paint for use in children's bedrooms, where you paint the walls, and they become magnetic so you can hang things on the walls. I thought I could paint things with it, like wooden tables, etc, to make them ~ non reflective for antenna testing. turns out...it did not help much at all.