Liz,
I would recommend looking at the parents of an iris with the feature you like, to find out how it was created.
I think one way to create an iris with a recessive trait is to cross both parents that have that trait. Sometimes a parent only needs to have the desired trait in it's ancestry to pass that trait on.
One of Kent's cuties Livy has parents that also have darts. ie:
I Must Have It
x Silk Run
Similarly, Buongiorno has parents with darts ie: Thundermaker x Stop Flirting.
I Must Have It has parents Romantic Gentleman x Stop Flirting. I don't think Romantic Gentleman has darts, but one of it's parents is Man About Town which has darts.
Have fun hybridizing. I look forward to seeing what you create.