After Rhaenyra Targaryen had won the dragon dance and finally sat in the throne she was born to be in; She must secure her position in it. After the execution of traitors, including the Hightower, and the Strong - two very major houses in the top 7 houses in the kingdom- one of the two new positions were given to house Blackburn. House Blackburn is one of the most respected houses with with great and high connections; it is one of the most trusted houses in essos and contributes to and owns importation and exportation ships. In Rhaenyra's eyes, and alliance with this family can help secure her position and help win over essos. This is how she came to the decision that her eldest son must marry the daughter of lord Blackburn as it would help her sons image and help him secure his claim on the iron throne as he is the heir. Jacerys x o/c