A simple way of describing Dark Arts is as the mirror image of (ir)rationality used for evil.

For example Eliezer writes about how our brains literally believe everything they're told, and are unable to filter out falsehoods while distracted.

The Less Wrong thing to do is to say, "Oh, better pay attention when untrue things are being said, so my brain can classify them properly."

The Dark Arts thing to do is to say, "I'd better distract people when I lie to them, so that even if they know I'm lying, they will still believe me subconsciously."