It is important to realise how the Black Madonna has a number of different meanings in this novel and is therefore a very important symbol. Most prominently, however, it is clear that the Black Madonna acts as a symbol of motherhood and the various surrogate mothers that Lily finds during the course of her novel. Note how Lily takes a picture of the Black Madonna with her on her travels, as it was a possession of her mother. In a sense, the picture is a symbol of Lily's mother, and is what guides her to finding her surrogate mother, August, who is able to give her information about Deboarah and her love for Lily. Note how this link is established in Chapter Three:
I was speculating how one day, years from now, I would send the store a dollar in an envelope to cover it, spelling out how guilt had dominated every moment of my life, when I found myself looking at a pictue of the black Mary. I do not mean a pcitrue of just any black Mary. I mean the identical, very same, exact one as my mother's.
It is of course significant that Lily is experiencing guilt when she realises that she is looking at the black Mary. As the black Mary symbolises her mother, and Lily still struggles with massive guilt issues regarding her mother's death, we can see how the two are linked.
You might also like to consider how the statue of the Black Madonna is also used to symbolise the importance of faith and belief in some divine power that is bigger than the individual. The way that the Black Madonna is used by August and the other women as almost a religious icon indicates the faith that they place in this figure as representing a strength greater than themselves.