Depends wether it abosrbs light or if light just passes through unchanged. If it's absorbed entirely, then it'll be black, if light passes through entirely it's be "invisible" (more like see-through since light will be slowed down and due to that there will be refraction). This should answer all questions adequately^^
Edit: The best example would be an airbubble in space, if light hit's the airbubble , it'll most likely pass through, but you'll notice a slight distortion(actually I don't know if a human I can notice it) which is the refraction. Depending on the strength of the refraction you might end up with multiple light rays of different colour.