Half-Saint's Light was indeed the soul of a Half-Saint.
When the Half-Saint was still alive, he could control the Divine Soul through his body. After he died, he could then utilize the blood of the Half-Saint and retrieve the Half-Saint's Light.
In fact, even if Zhang Ruochen obtained the blood of the Golden Cloud Half-Saint, it was not an easy task to reclaim the Half-Saint's Light inside of Lu Han's body. He would need to apply secret spell.
In general, only warriors of the Heaven Realm could assemble the Half-Saint's Light with the help of the Half-Saint's Blood. The reason behind that was that it would need to employ the power of the Martial Soul during the process of displaying the secret spell.
Only warriors whose martial cultivation had reached the Heaven Realm would be able to practice the Martial Soul.