According to ‘HMXOWL’ on the official Harmonix forums, let everyone know that the game will be taken down on PSN and XBL on OR BEFORE August 28th. See the post below in beautiful quote form:
After a good gap, it’s that time again. The demo and full game for Rock Band Blitz will be coming down on or before August 28th. Released in 2012 for the Xbox 360 and PlayStation®3, it’s not backwards compatible on either Xbox One or PlayStation®4(nor will it be in the future), but all 25 songs on the soundtrack can be brought over to Rock Band 4 at no additional charge. The steps for that process can be found here.
Speaking of the songs, all 25 songs featured on Rock Band Blitz were released as standard DLC after the game came out. Those versions of the songs will remain up until further notice.
Servers will remain up and intact for now.