Hoppers become darker on their bottom part whilst being broken.

Steps to Reproduce:
1. Make a new Creative world.
2. Give yourself a stack of Hoppers and a Wooden Pickaxe.
3. Switch your gamemode to Survival, place a Hopper, and begin to break it.
4. Notice the dark texture on the bottom of the Hopper. This can also be seen on the inside of the Hopper if you are standing at the same height as it.