Aurore, I hate to break the news, but you heard wrong. There are no blue dahlias. It's all a lie, like Santa Claus and the Easter Bunny. Until dahlia genetics can be altered, you're stuck with purple, pink, white, yellow, red, orange and blends or variegations thereof. Did I miss any colors?

Google these dahlias:
Whorton Blue Streak
Kari Blue
Blue Beard
'Blue Bayou' from Swan Island website: this doesn't look blue to me yet it is described as bluish lavender outer petals....
{{gwi:643407}}

I personally describe Kari Blue as blue-toned lavender, but it's still lavender ( and lovely, I might add!)
Those are the ones off the top of my head, but no blue such as you are seeking, sorry.

Poochella, you are absolutely right. There are no true blue dahlias today....maybe some day, but not today.
I'd like to add Thomas Edison to your list of so called blue dahlias.

Here is a link that might be useful: {{gwi:643403}}

Worton's Blue streak actually looks like periwinkle blue in my yard, has never been a very robust plant though.

Thanks for the list. Quite a few years ago I saw something on TV that was a lovely blue- maybe not a true blue, but close.

sort of like the legendary blue lagoon peony? LOL

Blue Lagoon is definitely a magenta! I was so disappointed when I grew it I thought it might at least be a grape purple. Too bad because it is really a beautiful dahlia and deserves not to be a disappointment!

Unless I've been fooled by my senses, the Dalia Blue Bell is quite close to a Royal Blue. I must admit that in nature, very few flowers are really blue, all of them will be closer to lavender or purple or even black; so when it comes to look for blue flowers we shall be a bit tolerant on this approach. Words from a Brit that is still working on a blue theme garden!
In Britain, Thompson & Morgan have them in Catalogue. Once they flower I'll let you know how blue they are.