TTL flash doesn’t aim for a single universal “correct” exposure for the whole scene. Like normal reflected-light metering, it generally assumes a midtone/roughly 18% gray reference and tries to set flash output so the metered area renders appropriately.

How it does that depends on the camera/flash system. Some systems meter a pre-flash, while others meter the flash during the actual exposure and cut it off when enough light has been received. Exact behavior is manufacturer- and model-specific.

Metering mode can matter. In broad-area modes such as matrix/evaluative, many systems try to balance subject and background. In spot metering, TTL may bias exposure toward the spot area/subject and ignore the rest more strongly. Some systems also use lens distance information to avoid trying to light a far background and blowing out a nearer subject.

So yes: if you meter a very dark or very reflective area, TTL can be misled, just like ambient metering can. A dark area may push flash output higher; a highly reflective subject may cause underexposure. In practice, TTL is useful but not infallible, and flash exposure compensation is often needed.