The most brilliant purple flower blooming right now is from the Purple Tiers lacecap hydrangea (Hydrangea serrata 'Miyama-yae-Murasaki'). The purple almost glows in the dappled shade. And it needs no soil amendments to keep this color from year to year. As you can see from the pictures, the flowers range from blue purple to rose purple on the same plant.
Unlike the oakleaf hydrangea mentioned yesterday, there is no question to this plant's heritage. It's a beautiful Japanese import that looks perfect against the native ostrich and royal ferns. Its blooms last briefer than some hydrangeas, but it is a wonderful color addition in late June/early July.
I never find this sort of plant locally so I ordered it from hydrangea.com. Have had good success with their plants and like their big selection. It arrived about 24 inches high and is now about four feet high three years later.