firebricks, come in two varieties. The heavy ones, which are made out of fireclay, and light ones which are refractory and typically have a high aluminum content.
I think as near as I can tell the difference between the black aluminum oxide and the white stuff, is possibly the black stuff comes from reprocessed Dross from aluminum melting. You take the dross whihc is white then, separate it, and remelt it to extract more aluminum out of it, and the black stuff is the dross from the second time around.
Now to add confusion, I believe alumina or aluminum oxide can have sodium aluminate in it, but I am kind of unclear what is going on exactly.