A dark order is a normal order, executed according to the same rules of any other order, but it’s not published on the orderbook, so only who create it can seen it.
It’s useful to avoid being overrun by trading bots (indeed, it’s not available via API) or to make big orders without influencing market.
Due to the fact that it does not help to show liquidity on the book or reduce spread, its trading fees start from 0.3% instead of regular 0.2%.