Affects Version/s: Minecraft 16w06a
Fix Version/s: None
Win 10 64 bit
Java 1.8.0_25 64 bit
So, that is a bit more complicated to reproduce and it is a bit weird.
Next to the steps in text form, I added a few screenshots to help to reproduce it.
First of all, if you press F3, there is one row with "Chunk x y z (...)"
You need to take care about the y-chunk-coordinate, which is the 2nd one of these
To do your setup for the bug, stack up until it says 0 when you are standing on top of your block (so the block itself is actually 15 of the chunk below it)
Then mark that block somehow and stack up another 17 blocks (the chunk y coordinate should say 1 if you are standing on top of the block). You can also stack up higher if you want, that makes no difference.
Now make sure, the block that is 17 or more blocks above the first one is floating and there is no block in between this and the one you marked before. Add a few blocks next to the bottom block (really doesn't matter, maybe do a 4x4 area or so).
If you are done, go back to title screen and reopen your world. There should now be a smooth shadow on the bottom block. If you stand where the shadow is, your arm or the block you're holding is getting entirely black. If you place a block on top of the shadow, the blocks surrounding it are getting black.