You use the verb "to be," which is rather deceptive in this situation (and in questions of optics generally).  The room itself would not "be dark" or "be light," it would be a collection of various particles, some of which would be photons in the visible spectrum.  It really only makes sense to ask how the room would appear to an observer looking through the viewport.

Even if you had completely reflective surfaces, the way the room appeared would depend significantly on the orentientation of the mirrors, the orientation of the viewing hole, the focus of the light, and perhaps most importantly, the refractive and diffusive properties of the mirrors.  Ultimately, the appearance of the room would be a result of the light that falls upon the viewport.  While there are an infinite variety of possible arrangements, the two extremes roughly align with the two possibilities you suggest.

For the room to look completely dark, no light paths fall on the viewport. A laser perpendicular to two parallel mirrors with the line of sight also parallel to the mirrors would produce this effect.

For the room to look completely bright, all (or however much you require to meet that definition) light paths fall on the viewport.  A room in the shape of a truncated paraboloid with the viewport at the focal point would produce this effect.