So if somebody decides to use an item such as Necklace of Fireballs and then plays a blessing, what die is added? The cards doesn't indicate which stat is being used...
The die the card uses gets added.
The dice added are normally of the type associated with the skill the character is using for the check; if a card instead specifies the exact dice to roll for the check, the added dice are of the type specified by that card.
So for Necklace of Fireballs, add a d6.