>With the flash in >TTL-BL mode, the camera tries to balance the flash exposure >with the ambient. It is anyone's guess what it will do in the >situation that you describe. That is why I never, or at least >rarely, use TTL-BL.
TTL-BL should only be used in bright ambient conditions for providing fill flash.
When using TTL-BL, the camera meter must be zeroed, so the ambient light makes a normal exposure. Then the flash adds fill to brighten the shadows. It works best when the subject is darker than the background, like when the subject is in the shadows under a tree.
Often the best way to use TTL-BL is to put the camera in P mode. Then, the camera meter is always zeroed automatically. In camera P mode the TTL-BL flash works together with the system to quickly make excellent fill-flash images.