Hi Ryan
Is this just mildew presenting itself on the perimeter caulk?
It's not uncommon that the soap & shampoo start to accumulate and/or the edges of the caulk start to loosen causing a cavity between the caulk and the tile giving the soaps a place to accumulate, then turn black with moisture and temperature.
Regarding the vent fan, it also helps to turn it on at least five minutes before showering to start air circulation in the room.