Is it a technical reason that almost all doors have the fade to black effect? Except a few doors in dungeons.
There are multiple doors which make the screen turn black when entering e.g. a building but at the same time these doors can be bypassed and players can switch between the buildings and the outside without a black screen.
Two examples:
1. From the last event, players had to enter a destroyed tower to pick up a wine bottle. Entering the tower through the door causes the screen to turn black. Once inside players can walk up the stairs and jump outside without triggering the black screen.
2. In Falkreath Hold, after defeating the skeleton boss players get a black screen when entering the catacombs, same when leaving them. But on the place in front of the house with the final boss it is possible to jump down back to the town area, bypassing the catacombs. Without a black screen. When facing the town, you just need to stay right of the stairs (don't go down), there is a spot where you can jump down. Its also helpful if you want to search for chests.