Royal Candles veronica (violet blue), Blue Hill salvia (violet blue), May Night salvia (dark purple). If you prefer a lavender rather than purple, nepeta would be good.

Deanna

My dwarf balloon flowers have flowers all summer long if dead headed. As do my campanula "Blue Clips." Others include russian sage, agastache "Blue Fortune", lavenders, and salvias. You can also use blue oat grass for a constant burst of blue all season long.

My perovskia (Russain sage) starts blooming in July and still looks good. I love it because all you need to do is cut it back in early spring and other than that it's pretty much maintenance free.

Salvia 'Mystic Spires'.

I love my purple lobelia, "La Fresco". Seems like the article doesn't agree with the noun, but that's what the tag says. It gets about 3-4' tall and blooms for a very long time in my garden. Hummingbirds love it too.

Scabiosa Butterfly Blue flowers all summer and fall for me even without deadheading. Agastache Blue Fortune, Black Adder, or Honey Bee Blue are all very nice and have blue spikes. Salvias are great too.

I love purple, some of my summer purples are:

Salvia 'Indigo Spires', Agastache rugosa, Salvia farinacea (annual for some), Salvia nemerosa (quite late for me) Limonium perezii, Verbena bonariensis, Delphiniums, Echinacea purpurea (more pink than purple) and Echinops ritro (more blue).

'Indigo Spires' is a great plant, really hardy and just starting to bloom again for me here in spring but it goes forever. I see others have recommended salvias, I must go and have a look at Mystic Spires, perhaps it's related.

Verbena bonariensis is a bit weedy but a lovely plant, maybe too pink for you but it gives great height.

Oh, and lavenders!

REgards
Susie

In addition to what people already mentioned:

Rozanne Geranium
Blue Satin Hibiscus
Crystal Blue Lobelia (amazing in full sun)
Russian Sage
Munstead Lavender (but it's really starting to bore me in the color dept)

Phlox 'Laura' is a good purple, is mildew resistant & blooms for a long time. Another good summer purple is Campanula 'Sarastro.' Shorter & more blue, but also long-blooming is Campanula 'Samantha.'

Oh my - how could I have forgotten phlox Blue Paradise?? Love that one!

Deanna

Thank you everyone for your suggestions. I am going to have to save them to study as I plan now and shop in the spring.

If you can find it, plumbago (the tropical one, the rest of the name escapes me), is the bluest bluest sky blue you can imagine. I had it in front of Salvia Mystic Spires (Navy blue and blooms ALL summer) this year, by accident and it has been incredibly lovely. Salvia, Black and Blue is great too, and take a look at Salvia Chaemydroides. A beautiful sky blue. If you don't mind annuals, Scaevola is second only to Salvia Farinacea in my book.
Also, just a note. Chartruese foliage planted nearby will really pump up your blues and make them pop. My favorites are Sprirea thunbergii, Ogon and Berberis, Aurea.

I agree with phlox and veronica, I would also rocommend violet, mini roses. People always seem to forget these. Most people buy them in pots and then throw them away after blooming, but you can put them in the ground and they will come back gorgeous every year! And they are cute too! You could also do some research and pick a purple daylily. There are some that do bloom a lot. I had one bloom for about 5 weeks streight this summer. Liatris is also a great purple plant, it has nice fringy foliage too. You got lots of great advice, hope you can find something perfect! -Jessica

But do you know summer goes from June 22nd thru September 22nd?

These are my blues and purples blooming between those months...some of which get deadheaded or sheered back for repeat blooms thru October:
Lavender
Echinacea
Salvia officinalis
Blue Flax
Hyssopus officinalis
Agastache foeniculum
Aster laevis
Snapdragon 'Black Prince'...yes, is perennial here (Fushia color). Leaves also become deep purple on undersides and tinted above and all purple in winter.
Nepeta mussinii
Rose Campion (fushia color)
Hollyhock (black...actually rich burgundy)
Joe Pye Weed (Eupatorium maculatum...deep purple stems and lavender blooms)

Also I have these purple/blue annuals in summer:
Petunia
Chinese Forget-me-Not....true blue)
Larkspur
Verbena bonariensis
Scabiosa atropurpurea 'Ace of Spades'....deep burgundy.
Moulin Rouge Sunflower
Castor Bean 'Carmensita'
Amaranthus 'Intense Purple'
4 O'clocks (fushia color....can save tubers over winter like Dahlia, but I'm also going to try mulching a few real well after hard frost kills them off)
Climbing Snapdragon (Asarina scandens 'Joan Lorraine')
Nigella

Vera

To all of these I would add my best new plant for this year, Agastache 'Purple Haze'. For me it is the best of the agastaches in the purple blue lavendar range. It has more of a pinkish cast than Blue Fortune or Black Adder but definitely still 'reads' in the garden as lavendar purple. It blooms longer and maintains a tidy neat appearance longer than either of its 2 cousins above. Tiny plants I planted out in masses this spring have been blooming hard since the beginning of July, and still have the appearance of being in fresh full bloom right now, when even the long bloomers Blue Fortune and Black Adder look a bit battleworn.

Verified z6 hardy, I am not sure we really know about z5, but here's hoping this absolute winner new plant will prove hardy for us.

Check out some of the blues on this page.

Salvia guaranitica ('Black and Blue' being one variety that was mentioned previously) will often survive into zone 6, especially in a well-drained and sheltered spot. I have a "Black and Blue' that has come back after each of the last 3 winters without any protection and is flowering abundantly right now.
Salvia 'Indigo Spires' should be treated as an annual above zone 8 but is a fine plant, and its blues get even deeper as the weather cools down.
Recently while in Santa Fe I picked up a couple pots of Salvia coahuilensis. It has small but vivid deep purple flowers. Hopefully the plants will do OK indoors under lights this winter.