welcome to GoY
How tall is it? to be honest there are many mid/dark blue ones and some have 2 names just to make it more confused. I have 3 dark ones of differing heights.

Steve Hickman holds the national collection and this is the link to his website. You may be able to match it to one of his.

https://somethingforthegarden.co.uk/

thanks about a metre i would say

Certainly looks like Navy Blue (I've grown it elsewhere before) but there are probably more varieties with dark blue flowers available now, so hard to say 100%