In fact, this matter is not so difficult to explain as some people say it is, and it is not so mysterious. It also does not mean the phenomenon of light spots that people see when they cultivate (Of course we have a friend who will disagree with me). However, it is a poetic riddle.   What Google translates as "black liver" should actually be "crow's liver" or "raven's liver". The reason Google mistranslated it is because the word 烏 has different meanings, one of which is black.    Saying "crow" or "raven" has nothing to do with the animal itself. It is just a symbol that the ancients liked to use. According to ancient legends, there is a crow in the sun. From the point of view of the inner alchemy, we do not need to know at all why the ancients thought there was a crow in the sun. This is a folk tale that has been in existence since the Warring States Period, and can be seen in the Shanhaijing and the murals of the tombs of the Han Dynasty. By the time Neidan was invented, it was already a well-known and ingrained belief, a bit like ancient Westerners believed that the god of war (Ares) lived on Mars. Because of this, the writers of the Song dynasty inner alchemy poems and books can use the crow as a symbol that the ancient Chinese reader would easily understand.   In the inner alchemy, the crow itself has no meaning. The authors of books about inner alchemy just used the crow to insinuate the sun, and then used the sun to insinuate the trigram of Li ☲.    For them, Li trigram represents the human mind. The two lines outside belong to Yang, so they are always moving, just like thoughts and emotions, which never stop. The one line inside of it belongs to Yin, so it is deeply tranquil. This Yin line is hidden inside, so it is difficult for ordinary ordinary people to discover it. But it is the innate, fundamental character of Li trigram, so it needs to be discovered. As a practitioner, we must discover this thing. It is the essence of the mind, which is basically Buddha-nature.    So why is it called "liver"? The answer to this question may sound a bit complicated, but it is also inseparable from the traditional symbols of China. The ancients chose "liver" to represent the Yin line of the Li trigram ☲ for two reasons:   First, the liver is an internal organ, so it represents an "internal" (or "innate") thing. But it is seemingly an arbitrary choice. Why liver instead of lung, heart or kidney? This is because of the symbols of colors.    Second, in ancient Chinese medicine, the liver is always associated with wood. In the Five Elements, the color of wood is green. Green was called 青 in ancient times. Well, the "green dragon" (青龍) was also another name for the Yin line in the middle of the Li trigram. Since in the five elements system, the green dragon and the liver are both considered to have the color 青, they both symbolize the same object in Neidan poems (that is, the Yin line inside of Li trigram, which represents our human mind).   You can see how the ancient writers used many symbols to say the same thing if read an essay from the Yuan dynasty in the Taoist Canon named 《紫陽真人悟真直指詳說三乘秘要》. It has many lists of these symbols, like a thesaurus or key to a cipher. You can find Raven Liver there if you search, it is called 烏肝.
Remember, the human mind is symbolized by the Li trigram. The Li trigram is symbolized by the sun. The sun is symbolized by the crow or raven. Layers upon layers of symbols, easy to get lost. As practitioners, we only need to discover the Yin line in the middle of the Li trigram. Doing this means discovering your own nature. It does not mean seeing some light spots when doing spontaneous Qigong.   "Rabbit marrow" is the Yang line in the Kan trigram ☵. It's a bit late now, too much to write. I don't know if anyone is interested, maybe I can explain it later. (Of course, we have to wait for our friend to say that I stink and there is too much Qi in my lower dantian (maybe if I have a fart there will be less Qi in the dantian, but this way I will stink even more, really don't know what to do )