NBA League Pass has local and national market blackout rules that restrict access to locally televised and nationally televised games for uses of the NBA Game Time Channel. A broadband blackout is determined by the zip code location associated with your Internet Service Providers (ISP) account. More information can be found here.
Blackouts are determined primarily according to who has the rights to broadcast a given game in a given area, and are defined by the NBA, not Roku. Blackouts take place when a game is televised locally in your area via a regional sports network or an over-the-air station. If a game is blacked out on NBA League Pass in your local area, you should tune to your local station to see the game. Games appearing on national television are not available on League Pass.
The NBA does not make blacked out games available on an archived basis. That is, if a live game is blacked out in your area, you will not be able to view it on NBA Game Time later.