I wanted to summon a fallingSand entity of stained glass (block 95) so I typed this command into a command block.
/summon FallingSand ~ ~7 ~
When I saw the glass falling, it was opaque and I could not see through it (I tried with many colours and they all had the same outcome). When I tried this with normal glass (block 20) it worked perfectly. In the picture you can see this happening on the stained glass block at the top.