Hello irtha,
Thank you for using Apple Support Communities. It's my understanding that your Apple Watch Series 4 appears blank when trying to use viewfinder in Camera. I can see how this could be concerning. I'm happy to help.
If you haven't already, make sure that your Apple Watch has the latest software.
Update your Apple Watch
If the issue persists, make sure that you're within normal Bluetooth range of your iPhone and restart both devices. Restarting can resolve many unexpected behaviors.
Restart your iPhone
How to restart you Apple Watch
Let us know the results. Please include any other troubleshooting steps you may have tried.
Have a wonderful day!