I'm intrigued that you had no problems with transparent materials. The issue I couldn't resolve was that they picked up an unwanted partial reflective property even when there was zero reflective level set. As shown by the right hand sphere in the pic above, there was a weird combination of reflection and transparency that was neither fully one nor the other. The only way I could get rid of the reflective element was to disable reflections overall in the render options dialog. This obviously meant that the other sphere would have lost all its reflection. Not very satisfactory.