Does this occur when going to and/or returning from commercials? This is probably caused by the commercials being at a different resolution and/or frame rate than the program you're watching. The Roku/TV combination must reset its video parameters for each such transition and this can cause a short blackout.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi LivRm - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi BedRm - JVC EM37T
Roku 3, 4200X, Ser 4124C7087583, wifi GstRm - Sanyo DP26640