Armor Stands Turn Black When They're In A Solid Block [1.12]

You might have noticed that entities are darker in darker areas. You can see that very clear on the arm of your player in first person mode, if you compare it in bright daylight and inside a cave. This darkness is to simulate the absence of light. The darkness of entities is determined by the light level of the block in which the feet are situated.

Now for your armourstand: The feet are obviously in a very dark spot, because the armourstand turns black. This is because opaque blocks don't transfer any light. The light inside the block is therefore 0. The armourstand reads this value in the block and will adapt its darkness accordingly.