Adds depth to Mercury, and connection between the conscious and subconscious levels of the mind.
Mercury can only gain in depth from its association with the Black Moon. The Black Moon adds sharpness and wit. Mercury allows an expression of the profound values associated with the Black Moon (which signifies the unconscious levels of the psyche). The combination can bring wisdom, as well as intuition. On a simple level, this intuition will result in instant and extremely accurate insights into other people's emotions - knowing their nature from the inside, rather than judging them by their facade.
The understanding of other people's psychology also has to do with the fact that the native's own inner life is quite sophisticated and complex. Mercury stands for the conscious mind, Black Moon for the subconscious and unconscious parts of the psyche. The association of the two means that there are more bridges between the different levels of the psyche than in other individuals.
This will not always be easy to bear. At times, this open communication between conscious and unconscious mind will trigger heavy emotions, crises, anxiety or even depression. Still, it is the foundation for a high degree of self knowledge.
On a higher level, a major aspect of Mercury to the Black Moon can help you become quite psychic (Clairvision higher mode). For Lilith-Black Moon, a highly metaphysical point in the chart, is plugged directly into your Mercury-mental consciousness. This means a natural affinity with the hidden mysteries of life, and a great capacity to fathom them.
Particularly for the conjunction and opposition, you will be well advised to watch the transits (passages) of Black Moon on the natal Mercury and Black Moon. These may be times when profound rearrangements of your thought patterns take place, and therefore golden opportunities to achieve a great amount of work on yourself. Conversely, the less you know yourself, the more these transits may result in situations of inner crisis.
Unsu...What I am trying to find and am great difficulty in is the aspect of Mercury being Semi-Square Black Moon Lilith. I would consider this a minor aspect of a major hard aspect. The semi-square being slightly less hard than a full square. If there is any kind of description of this particular aspect I would greatly appreciate seeing it.