It is not black. It is a deep velvet maroon-chocolate, the colour of crushed blackberries, and only at a distance does it pass for black. Close up there is plum and burgundy in it.
That distance effect is worth thinking about before you plant a row. Very dark flowers absorb light and read as gaps rather than colour from more than a few paces, so a block of 'Black Ball' on its own tends to disappear into a border. It is at its best with something pale hard against it: white ammi, cream cosmos, lime bupleurum. The same rule applies in a vase, where the contrast is what makes the arrangement.
The blooms are dense double Ball types, far more substantial than a wild cornflower, on silvery stems that cut well. It is the one to grow if the ordinary cobalt blue feels too obvious. Sow it in September rather than spring if the ground is workable. Autumn-sown cornflowers build a root system over winter and come up markedly bigger and more branched the following summer than anything sown in March.
