Thanks Quetscher. Lots of work went in to getting this far.
I think the dark sound comes from three main elements:
(1) The melodica itself; the Hohner Piano 26/27/32s are dark sounding to begin with if for no other reason than the sound holes project downward. If you play one upside down, it sounds similar to other melodicas, but does have the slightly darker vintage Hohner sound.
(2) The Lone Wolf Blues Company Harp Delay pedal; it is a must have for the blues. Not only does it round off the top end to remove harsh frequencies (making it sound darker), but as the frequency of a note changes during its duration, the echoes capture the original frequencies and the changes in frequency and mixes them together. I never play without the Harp Delay pedal.
(3) No doubt the biggest change in timbre comes from holding the sound holes against my chest. I can achieve complete muffling of the sound and choking of the reeds by holding the back of the melodica against my chest. The pressure and angle of the hold, all the way to a complete release provides an incredible range of timbres, including dark, dark, dark.
I will review my recordings and answer your other questions in a separate post.