Two subjects here, spot and ambient.
I am speaking of the camera meter here, which is a reflected meter. Incident meters are rather different.
The concept of spot metering ignores ambient, by definition. It is a spot, period. It does NOT see ANY of the rest of the image area. Beginners beware, Spot metering is anything but a general purpose point&shoot method. It requires some thinking and experience. Center and Matrix is more point&shoot.
Beginners imagine meters make exposure be "correct". But wrong concept, not possible. Instead the only concept is that the result is made to be a middle tone, not too dark, not too light. Regardless if it should be middle tone or not (black dress, white dress).
Any light meter averages its metering area into one exposure reading, and tries to make that reading's average come out about middle tone. Meters are dumb (cannot recognize the scene and what it needs, like human brains can). This is only way the meters can work. Center metering for example. Matrix metering too, which supposedly watches a few extremes too (in some undisclosed way).
Most scenes (landscapes, portraits, whatever) do tend to have enough range to average out to about middle tone, so this works, often (but some surprises, white dress and black dress for example). But a Spot is just a spot, and that spot area will be made to come out about middle tone too. Should a face be middle tone? (no, white skin ought to be about a stop brighter). So you have to choose your spot properly, or at least know how to compensate for that spot. Not for unthinking point&shoot.
Assuming we are not Spot metering, the relationship of TTL and TTL BL is that TTL ignores ambient, and TTL BL honors ambient. TTL BL (balanced) keeps the flash at fill level well less than ambient (if any significant ambient). TTL always does exactly what the flash meters, regardless of any ambient or not. If significant ambient is present, then TTL properly exposes, and ambient properly exposes, which is two proper exposures, and 2x is one stop overexposed (of the flash area). This is why we know to compensate TTL fill flash about -1.7 stops in bright sun. But TTL BL we don't necessary compensate (except for errors and choices), because TTL BL does this pull back automatically.
Spot metering only sees that spot. There is no concept of ambient. None of the rest of the frame is considered.
See
How light meters work