Then why does the brachial artery (in the wrist) spout bright red frothy oxygenated blood? Why does the radial artery (under the collarbone) do the same? Or the femoral artery in the groin?

Conversely, the jugular vein if cut will bleed dark, unoxygenated blood and so will the other veins.

Capillaries, for what it's worth, work the same way. The oxygenated ones are called "arterioles". You don't normally notice because when capillaries are broken they are so small that you usually break several of both sorts and the blood mixes.\

Which brings up the question of why Dracula would bite the jugular vein. No oxygen there. But then, maybe that's his idea of a quiet dinner and he saves the arteries for times when he wants a little more excitement in his food, sort of like sometimes you want a quiet sedate steak and sometimes you want habenero-scotch bonnet casserole.