Also known as Kurochi No Jutsu (Black Blood Technique), the Bloodline Limit of the Kurochi Clan removes all oxygen, nutrients, and life from the blood of the user, turning it black (and rendering the user essentially dead). The users DNA is sapped, giving them no genetic code.
The body can last only a very short time in the Kurochi State, because they are killing themselves by stealing the things that make the blood provide life. This technique is extremely dangerous because it damages the heart. The Kurochi forces the heart to pump blood with no health value whatsoever, so it is deprived of the things that it needs to survive.
However, this technique does have some value. By taking away the users genetic code, the user is able to absorb the DNA of another (note: there are 3 ways a Kurochi member can do this. A: Look into the eyes of their victim. This results in the weakest form of the technique, but it is the easiest. B: Touch the victim. This is in the middle, resulting in an average technique strength and average difficulty to use also most common. And C: Bite the victim the kind I have. This is obviously the hardest to do, but if you can attain it, it results in the strongest form of the acquired technique. Some Kurochi members have even attained twisted, further levels of techniques, named Heki. pronounced Heh-Kye Ex. Heki Sharingan But this is extremely rare and requires extreme power.) taking a piece of their genetic code, thus receiving the victim's strengths and abilities, such as their Bloodline Limit. (ex: If a Kurochi Clan Member released the Kurochi and absorbed the technique of an Uchiha who had the potential to activate their Sharingan, the Kurochi member would be able to activate the Sharingan.) This is why the Kurochi Clan has been named the "Blood Thieves"
The deprived nutrients and DNA of the user is not eliminated, just stored. This is why a Kurochi user has black veins, but red eyes. The DNA and life is transferred to and contained in the eyes, giving the user red eyes, because that is where the things that make the cells red are stored. So, when the Kurochi is re-contained, the nutrients and genetic code are returned, and the new code is merely assimilated.
After re-containing the Kurochi, the user can undergo 1 of 2 effects: They will have a massive explosion of power, or they will suddenly experience a blast of fatigue and collapse.
The first effect is the favorable one, where the user is endowed with an extreme power burst. This one is difficult to attain, because it requires total control. When the Kurochi is activated, it rapidly drains your life, but it also saps your energy. Someone, unless they are experienced at controlling the Kurochi, can't usually survive in the Released Kurochi State for longer than 2 minutes, because they will be too exhausted to move. Holding the Kurochi for longer than that will kill you. Therefore, being able to fight in that 2 minutes requires a great amount of physical stamina and chakra control.
The second effect is simple. You aren't able to contain the blast of power, so you get overloaded, pass out, and go into a comatose state. It's that simple. Now, as you can see, even if you activate the Kurochi, the chances of acquiring a bloodline limit and getting out of danger without dying are very, very slim. Most people of the clan have a lot of problems doing it. So, most members of the Kurochi Clan have been born with the natural ability to wipe the memory of people. Therefore, they will find someone in the night, freeze them if they don't go willingly, take their blood, wipe their memory, and disappear. This is how the clan can remain secret. Therefore, The Kurochi Clan has been associated with Vampires, and probably a lot of "Vampire" sightings were actually members of the Kurochi Clan being active.