Black Moon or Lilith is part of Astro-psychology :it is the moonspot in your radix which is most distance from the earth. It means you can not wear any mask or find ways of protection , -also subconcious levels ,you are completely 'naked' there, so most vulnerable. (How to Find All the Lilith's(= Black moon) in Your Birth Chart AstroFix)
Black moon Lilith symbol In present-day astrology, the name Lilith is usually given to a point on the horoscope that represents the direction of the actual moon's apogee, unrelated to the hypothetical second moon. When considered as a point, this Lilith is sometimes defined as the second focus of the ellipse described by the Moon's orbit; the earth is the first focus, and the apogee lies in the same direction. It takes 8 years and 10 months to complete its circuit around the zodiac.