I have imported a .obj into Blender and applied my own glass material to an object. This window pane then renders weirdly black in cycles. I built the same window pane in Blender with the same material and the glass came out fine.
Using an older version of Blender? In 2.74 the difference is quite prominent:
The “problem” is that since version 2.74 Blender does no longer recalculate (= average) vertex normals on every hit of the Tab key. This (Finally!) allows working with custom vertex normals (e. g. from CAD software) that are imported into Blender (Yay!). Unfortunately in this case those imported vertex normals seem to be wonky for some reason. Just delete them and all is fine: