Can anybody expound on the symbolism of breastfeeding the old woman in "Black Moon" (1975)?
In my opinion, (think of this as more a long comment than an answer), I don't believe that there is clear symbolism in that scene to expound on. That's not that there isn't any, but it's sufficiently convoluted that it's impossible to say. Symbolism doesn't have to be obvious, in fact, it's often best when it's subtle, but one thing symbolism should be is distinct so that, when pointed out, there's little disagreement on whether that scene is or isn't symbolism and what it symbolizes.
Examples: Dark/dusty represent neglect. Bright/Yellow, happiness. Pink, innocence. Red, more interesting, can represent blood or sex. Black can symbolize death. "Girl in the red dress" has one meaning "Girl in the black dress" another, "Girl in the yellow dress" a 3rd. Yellow Ribbon, Pink Ribbon, Black Ribbon, each of these images carries clear symbolism and when properly used, clear, agreed upon meanings. Now, not every black dress means death, it can simply mean fashion sense, but when properly used as symbolism, the meaning should be pretty much unmistakable.
On the breast feeding scene, there's not enough to go on to draw a distinct meaning of symbolism. No shortage of non-distinct meanings can be drawn. Even Malle himself said "he wasn't sure what the movie meant" and he implied disappointment in it. It was obviously more experimental than well thought out or planned. He called the dialogue in the movie "automatic writing" and unedited. Symbolism is rarely written in so free-form a way, it should be written by design, the specific symbolism (The sled in citizen Kane), crystal clear.
So, in Malle's own words,
he explains that the house in the film is meant to be a labyrinth leading the heroine into a “rite of initiation.”
Does the old woman symbolize death? Maybe, that's not uncommon, but the old woman comes across as more impish than you'd usually think a person representing "death" would come across. At times mean, at times playful. More of a teacher of sorts, perhaps a bad teacher, but she's one of the "rights of initiation" that the heroine deals with, and what does that tell us about that scene? There's some loss of innocence, and some feeling of duty and uncomfortable-ness. but I don't think there's any clear and specific meaning or symbolic message.
The film is a bit of a dream, so, what does that scene mean in a dream? Burden? Uncomfortablness? Introduction to sexuality from a parent figure? The Film Maker's fantasy? Who can say.
The movie is Alice in Wonderland inspired, so is the old woman the evil queen? Perhaps. The old woman at one point appears to make the young woman's panties fall down, or, maybe they fall down on their own and the old woman takes photographs of that. The Unicorn says "the old lady isn't real" . . . "Ofcourse she's real, I've talked to her". Drawing symbolism from all this, to me, is a big mess. There's no clear answer.
The naked children running with animals symbolize playful cherubs to me, and the movie has darkness too. Parts of the movie have clear symbolism but the scene you ask about, I can't make anything specific out of it. I don't think it's meant to be fetish in any way, but I do think it's meant to be shocking with perhaps a sense of duty and practicality (few things are more practical than feeding someone who is bed ridden), but obviously a sense of uneasiness too and the whole thing is shot in a rather elegant almost soft-core-porn kind of way - which, as I write it, seems full of contradiction but not meaning. The young woman doesn't like the old woman but she feels a duty to her.
The film is said by many to reflect the women's movement, but I'm not sure I get that. I do get a woman's initiation into something new and often unpleasant, and told from a dream-like perspective.
That's my take anyway. If anyone wants to disagree with me and explain the symbolism of the scene, I welcome that point of view.