Harmonix has just announced on the Rock Band forum that Rock Band Blitz will be removed from sale “on or before August 28th, 2017”. The game was released in late August of 2012 for both Xbox 360 and PlayStation 3 but unlike Rock Band Unplugged for PSP, Blitz never saw a physical release. Once it’s gone it’ll be Extinct.
Fortunately, all 25 songs on the Blitz setlist can be exported into Rock Band 4 at no additional charge. Harmonix has a support page here with details on how to make the transfer. You’d be best off going through the steps as soon as possible as the soundtrack export is technically a piece of DLC for Rock Band 4 and may also be removed after August 28th.
The 25 songs will still be available as standard DLC for Rock Band 4 and according to Harmonix “will remain up until further notice”. August 28th is the five-year anniversary of the game’s release and with the songs remaining as DLC it’s likely that the delisting marks the end of a licensing deal for Rock Band Blitz exclusively.