OK - I had been assuming that "dark arts" meant "use of techniques socially considered mean, nasty or evil

The meaning should be considered divorced from what is socially considered. Many things that fit in that category are not dark at all. In fact a lot of what 'dark arts' entail is doing things because they fly under the radar of social disapproval while still being effective. There are also many techniques that are socially considered mean nasty or evil while not being dark arts. Those techniques often also qualify for such labels as banal, honest, blunt and naive.

It seems the LW definition is more specifically about manipulation.

That's the one. With an emphasis on persuasion via manipulation. But the concept runs into the same problems as manipulation does in general. The line between 'manipulation' and 'actually having social skills' is basically non-existent.

I don't tend to use the phrase 'dark arts' myself. It's too cute, rather imprecise and places the emphasis on entirely the wrong thing - including things that are not even objectionable. I talk about bullshit instead.