Will seeds harvested be the same color as the mother plant?
As long as they are not a hybrid(not sure if there are any hybrid nigella) and not grown near other nigella cultivars, they will come true. If they are grown near others, I would suspect that the plants would end up becoming all blue over time since that seems to be the dominate color of nigella.
I'd be happy with all blue. All of the nigella six pack seedlings I've found are mixed purple, blue and white. I thought it would be fun to begin my first seed collecting and winter sowing by separating and sowing by color. In any case, it would be a good experiment.