When using the TTL setting, flash metering and ambient metering are separate. I believe that you are locking only the ambient exposure reading with the shutter half-press. To lock flash exposure, you have to use FV lock. 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.