I plant nigella seeds in November, they are already up now and have been for awhile.
The only thing I find about nigella here is that it is VERY very early, and that it is a bit floppy. However, like you I love it in flower arrangements for the pods. I grow a green podded type, several black pod types and the "transformer" which is just wacky cool!!
Nigellas will grow the same time here as larkspur, poppies, batchelor buttons, annual candytuft and the german catchfly. This makes a nice early annual garden.

GGG

My nigella last year was very, very floppy but very pretty. It was in rich soil but it didn't grow very tall. Volunteers are already up this year. I find it better for overseeding dying bulb foliage than for cut flowers.

I'd love to see your final list of flowers for cutting when you're done.

Nell