The question at the sanctuary in oblivion is 'What is the color of the night?' and the answer is 'Sanguine, my brother'.
I'm wondering, what's the relationship between the Dark Brotherhood and Sanguine? I would love to know, I've only recently been interested in the Dark Brotherhood.

That reminded me of Threads of the Webspinner.

So I would suspect it to be a lasting memory of the Morag Tong.

Otherwise, it should just mean: "By dusk, we spill blood."

http://uesp.net/wiki/Morrowind:Threads_of_the_Webspinner

To start, there are the Sanguine items you're sent to collect for the Morag Tong in Morrowind. They were made for Mephala by Sanguine himself.

Sanguine, as in the blood-red. The riddle is not to mean the prince, but the color/humor/maybe cheerfulness.

"What is the color of night?" with the answer being "Blood red", because you kill, usually at night, and your targets are going to be leaking a bunch of blood, which is a particular color of red. Also, as a killer, you obviously love killing, so it further goes down to mean "I [censored] love spilling blood at night!"

http://en.wikipedia.org/wiki/Sanguine is a reddish color, usually described as the color of dried blood. Hence, for the Dark Brotherhood, the color of night is the "dried blood" of their victims.

"Sanguine" is also one of the https://en.wikipedia.org/wiki/Four_temperaments suggesting a pleasure-seeking and sociable temperament (here referring to Sanguine's sphere of debauchery)

The word originally stems from the Latin word for sanguineus, meaning "bloody" or "of blood".

There are no known Elder Scrolls lore connection between the two, however.