Blood is not merely their means of survival. Yes, it sustains, infusing their cold, lifeless bodies with energy that could be gleaned from any other source.
But it is more than that.
It is life, captured and contained. Less a taste and more of a full-body sensation. It makes them remember, although whether the memories are really theirs or if they are simply imagined remains a mystery. Even drawn cold, sterile and inert from a sealed canister, it still possesses a unique power, though nothing will ever compare to drinking it fresh.
For a few brief seconds, their cold bodies feel warm, their own still blood remembers a time when it pulsed, their hearts constrict in the ghost of a beat. They take a deep breath, remembering a time when their lungs required the oxygen, when breathing was not simply a habit or a formality. They close their eyes, seeing for a second the brightness of the sun that used to warm them. All things they sacrificed, willingly or not. The price of immortality. It is the closest any of them will come to living, truly living, ever again.
It lasts seconds, or perhaps an eternity. It's all relative now, time. A minute means as much as a century to them, frozen in time yet pressed on mercilessly into infinity.
Vague? Yes. Short? Yes. Oh well. Hope you enjoyed it anyway!