black wont stand out as much as white. the black wont necessarily mean "fallen rider", claiming white as the color would. the bracelet wont be seen as prominently as the armband.
i truly believe white around the upper arm, whether it be a printed band or a bandana is the way to go. remember most rider jackets are black, white will stand out the most.
Black has always been a sign of mourning, like the black tape on police badges.
I think this is a good idea but maybe a patch is more practical. I believe is was Acehole who mentioned they where patches on their vests for fallen riders.