TTL-BL is not meant for indoors with dark backgrounds. I found that out during a wedding with my very first DSLR, a D70, and my SB-600 in a dark ark of a church. I shot a few frames, and saw in my LCD that my subjects were underexposed. I immediately switch to TTL and went on to shoot a successful wedding.
Since, as I understand it, the camera's meter plays a part in the exposure, since input is taken from it to set the proper flash setting to fill in shadows. In TTL, it is reading the output of the flash alone as the light strikes the sensor, or film if you are using a 35mm. Does that make sense?