I have not tried 4.15 yet… I am already having fun to test the new features in the google branch 4.14.
So well, sorry for the expression " pass the viewport camera of your first person character".
The black screen might be due to a camera that it not pass to the player controller properly.
Try to list all the existing camera in your level by using the node get all actors of class, I guess you will have 2:
Yours and another one created by the player manager, the issue you described seems to be that The UpdateViewTarget function is not done on the proper camera.
To debug this, You can also place a camera in the level and in the blueprint use the node " Set View Target with Blend" as shown in my attached png.
You should not get a black screen by forcing the camera as described.