The flowers have to turn into dried brown pods. In fact you can shake the pods and hear the black seeds rattle inside. They look like black sesame seeds. Yes, harvesting them is labor intensive. I just put the dried plant - stems, pods in a large plastic bag upside down, shake it vigorously then when I get around to it I cut a small hole in the bottom of the bag and the seeds and some residue falls into a strainer. But the harvest is always plentiful, constantly re-seeding themselves every year. Interesting, I just noticed the other day it is shown in my Bible in a few places. It's called Black Cummin.
Now isn't it just like our heavenly Father to tells how it's done!!
Isaiah 28… "For his God instructs and teaches him properly. For dill is not threshed with a threshing sledge, Nor is the cartwheel driven over black cummin; But dill is beaten out with a rod, and cummin with a club. Grain for bread is crushed, Indeed, he does not continue to thresh it forever. Because the wheel of his cart and his horses eventually damage it, He does not thresh it longer.…"
There is some residue that cannot be removed and I notice it in my Sweet Sunnah Black Seed Oil. Interesting that the Almighty tells us just how to harvest it. Whether you use it for retail or just pure enjoyment of the flowers, they are still a blessing that keeps coming up every year without my having to do a thing, and very easy to care for. Hope this helps even a little.