If you live in the Eastern US, then I might suspect a pollen/OAS type sensitivity to black walnut since it's native there. If the allergenic walnut proteins truly are the same or similar across all species of walnut, it might explain why she reacts to one but not the other.
TX - USA
DS - peanut, tree nut, milk, eggs, corn, soy, several meds, many environmentals. Finally back on Xolair!
DD - mystery anaphylaxis, shellfish.
DH - banana/avocado, aspirin. Asthma.
Me - peanut, tree nut, shellfish, banana/avocado/latex, some meds.