You can get quite fancy with a ventilation, but gravity set up works well, especially if you can position inlet/outlet on a diagonal or opposite walls (with darkroom sink in its path, so the draft takes any fumes with it easily). It all depends on actual room shape. Having no fan to deal with makes it bullet proof. If a louver is to communicate through an exterior wall, then you might also think of some way to control the flow, including ability to close it completely.