Say you have a bonfire, or a flaming sphere... and a shadow conjured aquaous orb rolls into the square thereof.
Aquaous orb normally says it extinguishes non-magical fires, and can dispel fire spells within its occupied squares...does its illusionary little brother do the same?
Shadow spells are only part illusion. Against something which can't be affected by illusion it has a % effect - 20% for shadow conjuration. In this case I'd read it as a 20% chance of extinguishing the fire.