My sister gave me some Nigella seeds a couple years ago. I didn’t know what nigella was at the time, but I was not going to turn down the opportunity to find out. The seeds I was given grew into plants with wispy thread-like foliage similar to fennel. They reached a little more than a foot tall with a single flower at the end of each stem. The blossoms were generally shades of blue to pale blue but some were pure white. The first year a majority of the flowers had five petals, while others had considerably more. Now after a couple years of growing nigella, it appears the flowers with many petals greatly outnumber the ones with only five.
After the flowers die off they are replaced with otherworldly seedpods. Once the pods are dry it is easy to collect the seeds and save them for the next year. But the nigella in my yard have been such good naturalizers that I generally don’t need to plant the seeds I collect, unless I want to start some in a new location. I have not found nigella to be invasive, most likely since new seeds are large and tend to grow close to plants from the previous year. Although they grow well in soil, mine have done even better in sand.