'pale blue flowers' - beautiful, as you say, Loiseau.
We lived in a cereal growing area and it was often a puzzle to guess what each field contained.
One farmer only ever grew maize - he fed it to his pigs. But the others chopped and changed, and occasionally grew flax. Other favourites were sarassin and colza. Wheat barley sunflowers etc.
I think flax was used as a green manure.