>TTL-BL, "Balance Light" attempts to balance the >flash with the ambient light. This is particularly beneficial >outdoors, where the ambient light is particularly bright. -BL >achieves this by altering the ambient exposure to better mesh >with the flash output. Some photographers like what it does >and others prefer to control the situation themselves via TTL >and exposure and flash compensation or by total manual flash >and exposure control.
I think you will find that TTL-BL does not alter the ambient exposure (ie, shutter/aperture). If you turn the flash off and check the aperture and shutter, and then turn the flash on and check the aperture and shutter again, you will see that the camera chooses exactly the same settings. This is, of course, assuming you are in bright ambient conditions, and using P mode, so there are no limits to the shutter speed.
>Indoor flash frequently presents a different problem. With >comparatively much lower ambient light (no sun), the flash is >dominant. Beyond the subject distance, the background rapidly >falls to black. TTL-BL, when it sees the big discrepancy >between the flash and ambient exposure, significantly >lightening the ambient exposure and making everything look >somewhat weird. When indoors, I recommend that you stick to >straight TTL.
Indoors, TTL-BL doesn't work well, usually, because the shutter speed is limited to Flash Shutter Speed. This is 1/60th by default, which will make the background very dark. TTL-BL will assume that the meter is centered, and it will add the appropriate amount of flash for that setting. If you were to put the camera in manual mode, and center the meter manually, then turn on TTL-BL you would get a good expoosure, but the shutter speed will be very low, and you will see lots of ghosting (unless you use a tripod).
The whole key to getting a proper exposure with TTL-BL is to make sure the camera meter is centered. That's what the TTL-BL flash always assumes, and it doesn't have any way to know if the meter is not centered.