Quote:
it is performed in the key of D-flat major, which is a very dark, mysterious sounding key

In the context of equal temperament, I'm inclined to call hooey on this sort of statement. I believe the sound of "Clair" has everything to do with its internal structure, the intervals and chord progressions used, and the physical situation with human hands playing piano keyboards, and probably nothing at all to do with the key per se.

Here, through the magic of MIDI, this is "Clair de Lune" transposed down a semitone, and so rendered in the key of C Major:

https://soundcloud.com/mark_hayes/clair-de-lune-in-c-major

Have I changed a "dark, mysterious" piece into something very different, something as "bright and familiar" as C Major? I don't hear it.

Now, where the whole piece sits on the frequency spectrum will clearly affect the sound — you would have a very different piece set two octaves up or down. But the idea that each of the 12 keys has some sort of musical personality, and in particular the idea that Db is a creature of darkness while C is a happy sheep in sunshine, strikes me as the aforementioned hooey.