If a block contains a separate segment, and it is located directly above an opaque block, the segment turns black. Some ways to find this are: Look from underneath at connected fences that are on top of blocks, put a slab in the upper half directly above a block, or place a block underneath upside down stairs.
(Also seen on iPod touch 4G)
Linked issues
duplicates 1
Comments 3
Extremely sorry about that, I searched but found no results. My post also mentions the slabs and stairs, but these are technically the same concept.
Is that related to MCPE-346 ?