You are quite right: "of which" is wrong here. If we re-order it we get, just one single blossom grows of a flower, which is not correct. Nothing grows of anything!
And saying, If someone loves a flower from which just one single blossom grows is horticulturally incorrect! Flowers are sometimes poetically called blossoms. Blossoms don't come from flowers: they are flowers. Whatever you call them, they come from plants.
We could say, if someone loves a flower, and it comes from a plant (which only ever bears a single flower and) which is unique in all the millions and millions of stars but that is rather wordy.
We could say:
If someone loves a flower and
there is no flower like it in
it is unique among
the millions and millions of stars. . .
We now have only the astronomical problem of flowers growing on stars. . .!
It's a much-loved book and perhaps the child's ignorance of grammar is part of its charm.
"He is her son, of which she is proud" means she is proud to be his mother. "Of which" refers to the fact that he is her son.
"He is her son, of whom she is proud" means she's proud of him.