Rothsay Reveler and Edinburgh are two dark purples (with white edging) that I know. Bet there are plenty more out there. Here'e Edinburgh and I can't find a photo of Rothsay, but I have one.
I think if you search the dahlia list for PR denoting purple vs lavender, you will find more to look at.

{{gwi:653207}}

Jennifer's Wedding is a dark violet purple:
{{gwi:653208}}

Willow's Purple Cactus without a name
{{gwi:649004}}

Camano Thunder is bright purple- wish I had one. Much more purple than this photo.
{{gwi:653209}}
Impression Fantastico, small collarette in deep purple and white
{{gwi:628221}}

Here is a link that might be useful: Some dahlia photos listed by size/color

Take a look at Thomas A. Edison. It's a wonderful dark puple.

{{gwi:653210}}

Oh Wow! Simply gorgeous! Now I just have to go hunting for Canadian dealers that I can get them from! lol

I would like to plant them with my Kelvin next year, so I'm searching ahead of seasons :D

Who'd a thunk Dahlias could be sooo much fun! And all the things you can do with them!

Welcome to dahlia Mania, ReddScarlette!

There goes evey spare cent you have

Happy Gardening, Willow

Here's another new one, a little pompon under 2 inches, more like one- great exclamation point in a bouquet in dark purple:
Wendy's Place
{{gwi:628953}}

Nikki if you like yellows and purples together- my personal favorite BTW- you should look into Polventon Supreme and Ivory Palaces- just lovely, and don't forget Nettie for that little zing of small yellow

Polventon Supreme:
{{gwi:653211}}

Ivory Palaces, so fluffy and wonderful
{{gwi:653212}}

I lied. Oh yes, bold faced lie. The above purple and white was Rothsay Reveler. Edinburgh has more white on tips. Here they are side by side.
{{gwi:653213}}

Ooohhhh VERY Purdy! I really like that Rothsay Reveler and the Thomas A Edison. I just LOVE them!

Ohhhh a hunting we shall go! For my garden sooo....High hoe the gardner oh a hunting we shall go :D

I found this one in my searches....holey is it every gorgeous!!!
{{gwi:653215}}

Now I just have to try and find one! ;)