What are the best places to get Dark Elf Blood in Skyrim?

I guess you are doing the second part to discerning the the transmundane quest. So Liars Retreat should get you most blood including the dark elf blood however or you could go murder a random stranger in the dark quarter. read more

A permanent corpse of a Dark Elf can be found at Evergreen Grove. At The Ritual Stone there is normally a Dark Elf Necromancer who can be killed and then have her blood harvested. Drelas Cottage, on the opposite side of the mountain to Morthal, contains a hostile Dark Elf mage named Drelas. read more

Three blood samples(Wood Elf, Dark Elf and Orc) can be found in the secret lair of Sild the Warlock in Rannveig's Fast. This is a small-size ruin and will never respawn. You can revisit there and collect the samples without any trouble. read more

The races whose blood you need to harvest are High Elf, Wood Elf, Dark Elf, Falmer, and Orc. Dark Elves, or Dunmers can be found in these locations: Liar’s Retreat, Rannveig’s Fast or Silverdrift Lair. Liar’s Retreat is an underground cellar with bandits inside. read more

In Darkshade, southwest of Windhelm, the body of a dead female Dark Elf can be found at the back of the cave near the bottom of the ramp that leads to a chest. A permanent corpse of a Dark Elf can be found at Evergreen Grove. Ustengrav has a scripted corpse of a Dark Elf conjurer on the stairs leading from the entrance. read more

In Darkshade, southwest of Windhelm, the body of a dead female Dark Elf can be found at the back of the cave near the bottom of the ramp that leads to a chest. A permanent corpse of a Dark Elf can be found at Evergreen Grove. At The Ritual Stone there is normally a Dark Elf Necromancer who can be killed and then have her blood harvested. read more