Shady Words™ can be ...

prefixed with "SUN" to make new words.

And they're called that because ...

right now, they're SUN-less.

You said my previous answer, prefixing with "SUN", was close (and actually worked until the Not Shady Words™ list was changed to preclude this answer).

While various dictionaries don't quite agree with each other about this new attempt, it is close to what I had before and most of the word lists I use agree with me so I'm going to give this a go ...

Shady Words™ can be prefixed with "UN" to form new words.

And they're shady because

they can be turned to the dark side (negated).