Starting with Minecraft 1.8, all blocks now use a block state system and grab their rendering information (such as texture, rotation, etc) from block state files located in a resource pack.

If a block state isn't found for the block and known metadata, the game defaults the texture to a purple and black square texture (commonly known as a missing texture in most games) - At least, it should. It doesn't as of recent snapshots - that's a big problem.