When I created a new world to play around in 1.9, I noticed there were glitched black blocks on the stairs of the village (Roof). The villagers were normal. You couldn't go in the glitched mini-chunk or else you would phase or something. The doors used the bottom only, and had the same glitched thing on their doorstep. It seems to be a new block. The villager jumped over it like it was dirt. When you opened / closed / broke the door, it would become normal. Also when you spawn the mobs on some houses that includes stairs as doors, they spawn inside! You'll then see it as a block.

Video -https://youtu.be/D2m4Wzxkf5k