I don't think transparency is the issue, in this case? I am not trying to contradict Agent Smith, but it doesn't seem to me to be the issue if it's just light that's not transmitting into your scene. BUT, these are the dots to which he was referring... In the Material Lab, are the channel dots for the transparency, or alpha... In this case, I just left it set to 'Blend Transparency', but either fuzzy or normal would work the same. They value and optics work off of the Alpha, or greyscale, middle channel for any given component. Except bump, which has it's own channel. The dot in the Color section for Transparency give it's shadows and internals the "A" component's colors... Blah blah, lots of words, pictures are better...