Write down what happens after the blink. Does the movie play normally, does the TV show a new HDR or refresh-rate label, or does it stay black until you change the input? Compare the same title twice. If a brief switch is repeatable at the exact start and the film then plays, record that separately from any later interruptions.
Share your source box and TV models and where the HDMI cable goes. A soundbar or receiver between them can affect the supported output. If your device offers matching controls, record the current values, change one setting, and repeat the same opening scene. Switch it back after the test if you still need the original format.
If the picture never returns, include the time, which input is selected and whether the TV's own apps still show a picture. That gives people a specific HDMI or device state to look at.