Hi:
I'm de-lurking for a moment from the Endurance.net.
The bloody shoulder (and also the bloody flank, to a lesser extent) was
a highly prised marking on desert arabs. Color genetics is responsible for
the marking, being that chestnut (or other solids) can be a "base color"
under
what looks like a solid grey. As the horse gets older, the color comes
through
more. Grey is actually a very complicated color at the genetic level, and
it's why grey has so many variations.
The arabs believed this was a special marking, given to a horse by Allah
and denoted everything they wanted in a horse: stamina, loyalty, speed and
toughness.
The ancients beleived color denoted alot of qualities. Have you ever read
the story of Kohli-Bey? It's another (old legend) about a bay arab of that
name. I read the book when I was about 12 and to this day, Bay is
my favorite color in any breed. (That book is probably out of print).
Barb Peck