I think I'd either try:

a. scorching another piece of the same wood to see if the surface could be uniformly darkened without destroying it, or
b. blackening the inner faces with something like india ink (after sealing the top!), or
c. dyeing the whole lot a darker colour, or
d. sand them some more - if they got too thin I'd consider adding a third pin with a bit of wider tubing or bead between the scales for a bit more rigidity.

Regards,
Neil.