Most royal families work whenever the father of mother has died that their child (If a girl of a boy) and their spouse will become the new rulers. So, whenever Mariam and Oritel decide to kick the coop, is when Bloom and Sky will become king and queen of Domino.
She might not be queen of domino. She might be queen of eraklion. Since she has 2 kingdoms to rule if she will become queen. So, she should be queen in season 6. But it's really unknown for now.