By Johnscott
Dumfriesshire,
United Kingdom 
I saw a hydrangea which has a very deep blue flower with a white edge and veining.
I haven't been able to see this in anyone's list.
Unfortunately I didn't take a photo.
Does anybody have any idea as to which variety this might be
12 Jun, 2018
Answers
Do you mean a white edge to the blue petals, or a white edge on the leaves? And was it a mophead variety or a lacecap?
12 Jun, 2018
Could you be thinking of the 'Cityline Mars Hydrangea'? The depth of blue is related to the level of acidity. Acid=Blue, Alkaline=Pink. It can be altered with aluminum sulfide which you pick of at the garden center. Results won't show up for a year or two.
12 Jun, 2018