Hardy geranium comes to mind right away. Later some blue salvias (annual or perennial), Speedwell, pansy.

If there's enough sun, annual lobelias would be great for long-lasting intense blue. For blue perennials I think of blue delphiniums, blue bell flowers (campanula persicifolia), and blue balloon flowers.

There are more blue-purple flowering perennials than true blue ones but I'm guessing you want real blue. I have perennial blue flax, delphinium, and Jacob's Ladder that bloom for a fairly long time here and are true blue.

There is a hosta with blue leaves if you need a plant for shade. Also a blue sea holly I'd love to have but it's not for my zone. Also columbine comes in a blue, and grape hyacinth for very early color.

Probably easier to use several blue flowering perennials that would have successive bloom times. Don't forget about the silver foliage plants as they go very nicely with blue flowers and last all season.

Caryopteris, Perowskia, Omphalodes, Brunnera ...

Plumbago is a beautiful blue, but it is an annual here. I always use it in my pots on the patio and deck.

Salvia uliginosa (Bog sage). It is a beautiful blue, long lasting perennial. Looks beautiful swaying in the breeze and bees love it!

Thanks everyone! Lots of beauties to plant next spring. I looked them all up.

Perennial Plumbago - Ceratostigma plumbaginoides.

veronica 'royal candles' has bloomed most of the summer for me (with deadheading). super plant--love it!! :-)

I'd recommend annual salvia. I think mine is called Victoria Blue. I collect the seed every fall so I haven't had to buy for several years. It's easy! And like all annuals, blooms most of the summer.

Another salvia! Salvia guaranitica 'Black and Blue' reported to be a magnet for hummingbirds and butterflies. Gets big, will need a good bit of space. Supposed to be hardy in zone 7 but suggest you get it early next spring if you want it and let it get established during the summer. A new plant might not take the winter but an established plant should be fine. Different salvias will give you a variety of looks as their habits are quite different.

I have not grown this one myself but got 2 other guaraniticas this summer after reading praise of this type of salvia in hummer and butterfly forums. My plants are still quite small but are doing well. Good luck!

Here is a link that might be useful: Black and Blue at Almost Eden Plants

The longest-blooming true blue here is plumbago. I don't know how it does in your climate, and it gets very, very big.
Renee

In this picture dwarf blue delphinium (butterfly blue), and blue globe thistle in back. I don't mulch this bed and originally planted one delphinium. You can see it reseeds, which I like. Sometimes in lighter shades also but true blue. It has a very long bloom season. One plant is still blooming and we have had a couple of light frosts. I also like salvia black and blue and salvia blue spires (not hardy to my zone). I haven't been able to over winter it here but am attempting this year in a more protected spot and mulch. If not I will purchase again--its worth it. I also have the tall blue delphinium. As mentioned silver foliage looks great with blue. Lambs ear, artemisia and lungwort "Diana Clare" which has blue flowers in spring.

The annual plumbago is bluer than the Ceratostigma, I think. I also like the form better, but the perennial one is a good ground cover.

I agree that Victorian Salvia, mundane as it is, gives a great spot of blue and frequently survives the winter.

Add yet another vote for Salvia Victoria. It is a perennial for me and blooms all through the growing season. It also readily reseeds but can be easily removed if you don't want it to spread.