My Apple Watch's screen is blinking, how can I fix it?

Does the watch restart after the display went dark, or does it still show the actual watch face and works normal, until it shuts down again?

In that case this could be hardware related and the battery has to be replaced.

Getting the watch diagnosed by visiting an Apple Store or Authorised Apple Service Provider would be the the first step to find out more.

Apple - Find Locations

Apple Watch Service and Repair - Apple Support

If it restarts, I think, that at least the battery needs to be replaced.

If you visit an Authorised Apple Service Provider or Apple Store next to your place, they could also diagnose your watch.

Apple - Find Locations

Any error messages or icons shown on the display?

Has the battery been replaced already?

Have you been swimming with your Apple Watch lately?