Only through extreme emotional and physical pain may the use of the Hikari Clan's Blood Style. Such is why the Hikari put their own children through torture when they turn three years old. The torture continues until they are eight; it is then that they are placed in an arena and are given a special poison that Tayagane created herself. It is through the stress of their life clock ticking away that they begin to get desperate, and only in a moment of utmost desperation and hopelessness can Blood Style unlock.
With this Kekkai Genkai, you can control blood, but only your own. Developing the Kekkai Genkai comes with extreme physical and mental strain, usually followed up by torture throughout your entire childhood. Once an entire class of children is deemed mature, they are given poison and placed in the Arena. The one who successfully uses Blood Style gets the antidote; those who kill more than two of their peers are deemed as a success and are placed in the ranks of the militia. Those who are not deemed as a success are placed back in training, or dead. This strain is necessary, as it helps the children of the Hikari deal with pain, making their aptitude with Blood Style increase and helping them draw blood without hesitation.
To increase blood production, the Hikari can speed up the process with their chakra, although it puts them in danger of a heart attack. In order to avoid this danger they often use blood pills. They can also store blood in their hair, causing pain if the hair is cut. There are thousands of mini blood vessels that dye their hair purple. It is extremely common for users of Blood Style to have their hair down to their ankles.
When a Hikari reaches prime strength, they can possibly add a trait to their blood depending on how they were trained. Acidic properties, elemental properties and more are available. Arteries and blood vessels of a Hikari's body have an increased durability and are able to withstand more than the average person, plus they can regenerate said arteries and blood vessels very quickly.The Hikari can also speed up, or slow, their bloodflow to cool or heat their body, therefore making the effects of temperature useless against them.
Skin may be reinforced by hardening blood underneath it.