You can use StartCameraFade for that. On Begin Play, you set it to fade to black (Alpha=1) instantly (Duration=0.0). Then, when you are ready to fade in your movie, you do another StartCameraFade going this this time from Alpha=1 to Alpha=0 over a suitable Duration, say 0.2 or 0.3 seconds. That’s it. Here is a reference to the node:
Other techniques involve placing a black sphere around your VR camera, then changing its opacity over time until it becomes fully transparent.