According to the physics Smack My Pitch Up should be the ultimate 'dark' sound (pure sine wave). But my brain hears mid-pitch piccolo and tells me it's one of the brightest colours in the orchestral texture. My own low C has far more harmonic/overtone complexity in it than my high C, yet when I hear it, my brain says 'dark'. In fact 'dark and somewhat raucous' unless I dull it down a bit with a less energetic buzz. No idea at all what a 'bright' low C is supposed to sound like. Loud and sharp I guess.
What other people are picturing when they think 'dark', I just don't know. I remember posting the opinion that one TMer's clip sounded like he was playing down into a thick carpet. It wasn't meant to be a criticism, but I think he was a bit offended. His 'dark and sexy' seemed to correspond with my idea of 'dull and lifeless'. Maybe it's as much about our different musical backgrounds, expectations, associations and preferences that lead us to give positive or perjorative connotions to different sound textures and the words we use to describe them.