I have found another cause of this, which was producing all sorts of mayhem, like alphas being applied in one renderer but not another, and blended quads going black only if they weren’t stacked over others, etc. etc.
The culprit was a FileTexture (DX11 2d) node that had a file specified that did not exist. Apparently the non-texture it was outputting as a result when applied to a quad screws up the blending in the render.
So check your FileTextures too! Sounds like DX11 needs to handle absent textures a little more gracefully and/or throw a better error message.