Try increasing the number of reflection and refraction rays in the MR global attributes. If you have too few rays in there glass will show black...This usually happens if you have two transparent surfaces next to each other, or the same surface but bent so that light travels through 4 layers of glass...for each pass light makes on either side of the glass you'll need 1 ray. Try bumping the number to 6 on both reflection and refraction, the ray depth should be the sum of # of reflaction and refractions, which would mean 12 for ray depth if you used 6 on each. You rendering will take a ltl longer thx to all this extra work...