It uses a fade to black script meant for cinematics. I'd assume the script to remove it is tied to the teleport itself, so if you've interrupted it by continuously moving because (I believe without being able to look at it personally) the script uses an "ActionJumpTo" type command instead of just "JumpTo", then I'd say this is why its happening.
The result of the teleport being added to the end of the action queue instead of the front means you can end up cancelling it if you're in combat, casting spells or performing any kind of action that offers a chance of being able to clear the queue before all actions have resolved.
I suspect theres either a delay to return it out of black, or put into the OnEnter of the area you're supposed to be going to, or OnExit of the area you're leaving. How long does the blackness persist for?