Stroblianthus dyeriana.

Oh! Nigella! That's beautiful! I've never seen it before.
I'll pick my 'The President' clematis--at least it looks dark purple to me,And it's already had ten blossums ,with eight more coming on.
OhOh,that probably doesn't qualify for the question!!Oh well.
I don't think I have a dark purple perennial!!

I forgot my irises!!!!!!!!!1

Zone envy! Persian shield is not a perennial here, but mine came inside. I love it so much I got another one last week and put it right in the ground. You're right-on with this one, nigella! Do yours ever flower? The potted one I've had for a few years makes a flower once in while inside, but not outside. Do you know what triggers them to bloom?

I saw some beautiful dark purple columbine this afternoon, too. Love it!

MMM, there are a lot of things that don't grow well here, it's a trade off. I usually(I haven't gotten around to it this year) pick the blooms off, they take away from the vigor of the leaves, which become very sparse and thin about the time it blooms in the spring. I wish I could be more help.

Malva sylvestris 'Bibor Felho.' Like a hollyhock on steroids. Great deep purple-black colour and it blooms all summer.

Little purple violets that have violet tinge to their leaves. They bloom all summer in the shade if I keep them deadheaded, but if I don't, the foliage is very nice. Great groundcover and charming. Pest-free.
Spread fairly rapidly and easy to control. The perfect plant.

(How's this for a delayed reaction? LOL!)

Thanks, nigella. :+)

The malva you mention sounds neat-o, ninamarie. Thanks.

paulyn, I don't think I've seen a violet like you describe. Would you know it's latin name? Sounds sweet! I LOVE violets of any kind.

Heliotrope, here. Luscious plum purple, wonderfully scented. Blooms year 'round in my garden.

Omg you mean heliotrope is actually a perenial? So, if i took the heliotrope inside for the winter (zonal deficient), I could have it once again the next spring?

Mispelled "Perennial"! Gosh! I knew the preview was for a reason!

Preview - LOL! Yes, heliotrope is really a tender perennial. I knew I was asking for major zone envy with this question. Oh well, that's ok. I wish I could afford to have a big mass of heliotrope!

Heliotrope is usually sold inexpensively as a bedding annual in the spring. I hate to admit it, but the white usually seems much more fragrant.
The purple varieties are easy to raise from seed, but tend to lose their fragrance.
But you can strike cuttings from both the white and purple fairly easily.

Delphinium.

I'm new to gardening, but I found a purple clover oxalis (I think that's right). It's especially pretty when the lilac flowers are blooming.

Not really a perennial, but a tender shrub, black knight buddleia. And I love mealy cup salvia. A perennial in my area.

I must confess a weakness for my purple balloon flowers. They are so cute just before opening, then they pop! into that starry shape. They are easy-care, and bloom all summer in my garden.

Another vote for irises. A close second is my penstemon whippleanus blooms. Very dark purple and they hang gracefully downwards. But what would late spring be without purple irises.

salvia caradonna; euphorbia dulcis 'chamaeleon'; sedum caucasicum 'matrona'; and allium phaerocephalum, oh and iris pallida 'claret' and iris pumila 'little dream'. I guess some of the first plants are more burgundy than purple, but they share the same palette.

I don't know what it is called- i think it is a kind of wisteria that is so dark purple it is nearly black, with a more burgundy base. It stinks really bad though.

Has anyone tried Sedum 'Purple Emperor'? Incredible deep, somewhat glossy purple foliage. Mine is just about to flower, but if it never did, I'd still love this plant. Possibly the best purple I've ever grown.

Hmm... I wonder if that's the broken piece of Sedum I 'rescued' while shopping a couple mos. ago...? Very purple foliage.

Great ideas from everyone!

Campanula 'Kent Bell', very long blooming. Lovely plant.