Is this amulet of blinding fire balanced?


I created the following item with a 5th level draconic (red) sorcerer in mind:

Blinding Fire Amulet

Wondrous item, rare (requires attunement by a spellcaster)

The amulet has a number of charges equal to the highest level of spell slot available to you. It gains all expended charges daily at dawn.

Whenever a spell you cast deals fire damage to a creature you can expend one charge to force the creature to make a constitution saving throw against your spell save DC, or be blinded for one minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

I thought about the following modifications:

  • Make it work on a spell rather than a single creature, e.g. all creatures damaged by a fireball. I am pretty certain, however, that I would need it to require multiple charges for this effect.
  • Restrict the effect to spells of first level or higher. Not sure, if this is useful since there is already a limitation through the charges.
  • Reduce the duration of the blindness to one turn, instead of (up to) one minute.

Question

Is this item abusable or is its description ambiguous? Is it wildly appropriate for the suggested level or rarity? Would one or more the of potential modifications indicated improve the item with respect to any of these points?