It's a potion that can only be produced by land owners and sold on non-card market to anyone (basically, a precursor to items and spells). It doubles chances to get black foils in Conclave Arcana packs. BF is a new type of foil that is at least 3x better than GF (that's a conservative average based on its benefits compared to GF).

It's understandable that you don't pay enough attention... Not as much at stake for you anymore.