|Cassie, Diana and John|
The relationship that John Blackwell had with Amelia Blake was a marriage so they have a daughter who's name is Cassie Blake. The relationship that John had with Elizabeth Meade was an affair because they were both married to another person. John was with Amelia and Elizabeth was with Charles, so the daughter that John and Elizabeth had is an illegitimate daughter named Diana Meade.
In Return, Cassie first met John at her house and later met with him again to talk. Diana first saw John while trying to save him from Eben's spell on Cassie. John stayed to help Cassie and the circle against the witch hunters.
In Crystal, Royce Armstrong revealed that John Blackwell wanted children from the circle and that Cassie isn't the only child of Blackwell. Cassie tried to help Adam and Melissa but couldn't due to the spell that keeps witches with dark magic from entering. Diana told Cassie that she couldn't help them because of the same spell Cassie felt. Cassie was able to figure out that Diana is the other Blackwell in the circle and is her sister.
In Traitor, Cassie lost her grandmother and Diana came to comfort her while dealing with the fact John is her father. Diana went to talk to her dad Charles about her mother's relationship with John. He advised her to stay away from John and to not trust him. She convinced the circle to get the crystal back without John knowing. Cassie and Diana were talking about how Cassie is losing sight of whats right due to their father's ideals.
In Prom, both Cassie and Diana find out why all the six members of the circle were born at around the same time and John's true intentions are revealed to them, which causes them to lose their trust in him completely (especially Cassie, since Diana didn't trust him in first place).
In Family, John was able assemble the crystal skull with Cassie and Diana's help. John however has a different agenda in using the crystal skull and knock them out cold. He reveal his plans to his daughters about using the crystal skull ridding the world of witches except for the Balcoins and to form a circle of Cassie, Diana and his four other children. Cassie and Diana were able to free themselves and used the crystal skull to kill John once and for all.
- The two daughters of John Blackwell: Cassie and Diana, both lost their virginities with Cassie's mom ex-boyfriend and real love's son Adam Conant.
- John has a total of six children, Cassie and Diana from the Chance Harbor circle and four more from the other two circles.