thanks for the clarification. I didn't say blood was blue, only that the
viens are blue.

Jeff

Not just by skin. I can tell you from much direct surgical experience
that many isolated veins, not covered witih skin or even vessel sheath,
are quite blue when filled, white when not (so it's not quite true the
veins are blue-- only filled veins are blue). But the blood within them
is indeed dark muddy-red, not blue.

Some of this may have to do with the thinness and structure of veins
and skin, vs the relatively much thicker and more muscular arteries.

Here's something interesting: it's possible for veinous blood to be
quite red in a well-oxygenated animal, and yet the veins STILL blue.
For example, I just saw a few drops of blood drawn from an external
vein (probably an external branch of the femoral vein) in the rear leg
of an anesthetized cat breathing 100% oxygen as part of anesthesia. The
small blood sample (0.2 mL) was taken for antibody/virus testing. Such
blood samples from this area are generally dark when taken from cats
breathing normal 21% oxygen air, but in a very-well-oxygenated resting
cat, such venous blood may be quite red-- nearly as bright red as
arterial blood. Yet the vein it comes from, remains blue anyway!  In
this case, albeit seen through skin.

The cognoscenti may wonder how it is that venous pO2 at all when 100%
O2 is breathed, since oxygen content of arterial blood doesn't go up
that much. But there's some normal A-V shunting in the drainage of the
heart and lungs which affects venous pO2, and also there's a lot of
shunting in resting muscle, such as in a limb. Anesthetics affect
shunting also. So not all venous blood has been though a capillary bed.

SBH