Ads deepth and consistency. The spot where it's applied is rendered entirely opaque. It comes with a weight of it’s own in every composition. Behind black you don’t expect to discover anything. In deep black anything can be hidden. It is the master of obscurity. It moves subtly un-noticed between layers of colour. It is the master mediator ballancing a composition or setting it in motion.
White is extreme light. It can be incandescent and burn you or it can be cold and freeze you. It can frighten or it can calm, depends on the context. white is everything and nothing at the same time. It freely offers very much, demanding very little. White intimidates. It imposes purity. It is also somewhat fragile being easy contaminated by it's surroundings which are very likely to not be white.
Both are somewhat artificial and natural. A cloudy night sky is deep dark. The snow freshly layd is white. Both are ephemeral.