Black Box income is non-artist/title specific income (such as audit settlement monies, unclaimed monies, leftover amounts from an unrecouped advance, etc) from digital music services.
This income is paid to AWAL based on the usage % share of its catalog on a particular music service. We then complete a pro-rata calculation based on your performance compared to other AWAL/Kobalt clients on that specific service.
Black Box income is generally only paid once a year and will be shown on your PDF statements as an adjustment. The description will be along these lines, from a recent example, "Spotify Blackbox October 2016".