Lets see, Darkness is an Evocation spell that makes an object "radiate" darkness. Illussion spells are those that alter percetions or create false images. If you rule that dwarves can't see through illusory darkness, then they will immediately notice the anomaly and know it's either an illussion or a darkness spell.
Will your illusion spell allow a save for disbelief?
If no, then what's the point of having it in addition to the Darkness spell?
If yes, for a dwarf, is the fact they can't see through it enough proof to automatically disbelieve?
My point is: Why mess with "illusory" darkness if there is already an evocation that creates darkness? It's more trouble than it's worth. Just use a Darkness spell.