In addition to being unable to fulfill your main goal there, the BF is basically a dead-end, with no reason to stay permanently.
Travel between the Rivellon and Nemisis universes is done by opening rifts between them. The BF is a small pocket universe, which does not require the riftrunning skill to reach, since there are special artifacts (keys) that can give you the ability to travel there and back.
After getting out of the prison level, you start finding evidence of the previous inhabitants (the statue of Raan, etc), so it would be unusual not to say something, or be able to ask others about them. Riftrunning was brought up in the opening conversation with the DK, though not by name. IIRC he goes into more detail at the very start of act 2.
