Changing the "Item" nbt data of a block with /entitydata that has a damage value (like potions) will cause the block to become un-textured. To fix it I assumed you could just edit the damage value back to 0 by adding ",Damage:0s" but that does not yield any results.

Here is a video showing it in action: http://puu.sh/aJT5u/03155b3c9d.mp4