While I'm not entirely sure if this is intended, it doesn't make sense with how this entity functions. When a block, item, or text display is placed within a block it causes the display to turn dark like a mob. Seeing as these displays are meant to replicate the look of physical game objects it would make more sense if they didn't turn dark.

Here is an image of an item display positioned one block above the block it represents. When another block is placed on top of it (inside the item display) it turns dark.