There is currently a bug where if you are unconscious and respawn, on your next spawn in you will have a black unconscious style overlay applied to your newly spawned character until you die again. This has been incredibly prevalent on smaller more pvp focused maps where players are going unconscious and dying frequently, such as Namalsk. It is however a vanilla bug as evidence by the attached video taken on an Official server. I started seeing a significant increase of reports of this happening after the DayZ hotfix that was applied on November 3rd, however it may have been present before then.
The attached video shows me breaking my leg and going unconscious. While unconscious, I choose to respawn and then have the black screen bug upon spawning on my next life. This is not a 100% reproduction. This time it took my probably 20 respawns before it happened. Other times, when not recording, I had it happen much more quickly.