Congratulations on gardening with the lovely Nigella or Love-in-the-Mist plants. This cottage garden plant will reseed and the airy blooms and foliage combine well with other flowers. You did not mention your climate zone but if all danger of frost has passed you can transplant the seedlings outdoors immediately. If frosty  nights are still a possibility, I would transplant outdoors anyway but cover the plants with plastic milk carton jugs with the bottom cut out at night or some other type of covering to protect from frost. The young plants transplant best when small so do not keep them in pots too long. This plant likes consistent moisture so do not let the plants dry out but do not overwater especially if weather is cool. They adapt to most soils with good drainage but bloom best in full sun.  Be sure to leave some seed heads in the fall. You may be welcoming new seedlings popping out of the soil next spring as this self seeder has many happy returns. Keep growing, Marianne Binetti