Nokia’s Lumia Black firmware, which will come along with Microsoft’s Update 3 OS bump, is due in "early 2014". It seems however that the Finnish company wants to make sure all is right with the firmware as they are offering customers early access to work out any bugs.
The proposal seems to be tied to using the ‘Feedback to Nokia’ app found under Settings. Users who have evidently competed the mini-survey are being contacted via email with the offer to test Lumia Black before anyone else gets it. The early-access beta program appears to be linked to Lumia 520 and Lumia 521 devices, although it’s not clear if other Lumias will have the same testing program.