The title pretty much says it all. When a FallingSand entity is touching a solid block from any side, including above and below, that side will turn black. See screenshots below.

How to reproduce -

• Summon a FallingSand entity. I would recommend summoning it riding an ArmorStand so the effect is easier to notice.
• Place a block directly adjacent to the FallingSand so that the sand touches the block. This effect does not happen when the top of the block is above the top of the FallingSand.

If additional screenshots are required, just say the word and I'll get more.