You can attack with Crown of Stars and cast a spell with your action
Crown of Stars takes an action to cast, once cast it grant a bonus action ranged spell attack. This attack does not count as casting a spell so there is no limitation on what you can use your action for. The relevant part of Crown of Stars is below (emphasis mine):
Seven star-like motes of light appear and orbit your head until the spell ends. You can use a bonus action to send one of the motes streaking toward one creature or object within 120 feet of you. When you do so, make a ranged spell attack. On a hit, the target takes 4d12 radiant damage.
The restriction you are concerned about is the bonus action spell casting rules:
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
Making a ranged spell attack (which Crown of Stars requires) does not count as casting a spell. Therefore this rule does not come into play. Even if it did you would still be allowed to cast Eldritch Blast as it is a 'cantrip with a casting time of 1 action'.