Is chocolate your only sugar intake?
Dark vs light really matters little. It's how much you eat, of it and sugar in general, if we are talking just the sugar part. Sugar goes hand in hand with fats, whether dairy or plant, and that's another part. Chocolate of course is cocoa fat and sugar. If you eat 1 or 2 oz a day, fine.
Red meats and animal fats are part of this because of cholesterol, which is a factor in BP. You need only 3 or 4 oz of concentrated proteins a day, and many argue that even that is more than enough. If chicken and fish are your main proteins, stay under that amount of oz.
Exercise keeps the blood circulating, the vessels elastic, the fats burning, and air in your lungs and the exchange of gases for cell health.
SMOKING is a big part.
Certain medications - even birth control - may play a part.
Heredity is the rest.

Great minds think alike cause I say it all the time. :D

Dark vs light or milk chocolate I would think would vary by brand. For example Hershey's chocolate contains between 4% and 10 % chocolate. Its mostly sugars and other fillers. Whereas Lindt's is 70% chocolate. Caco or cocoa with very little sugar is healthiest in the long run. 5 Reasons Why Dark Chocolate Is Better than Milk Chocolate

I should have said that dark vs light matters little in the overall picture. She isn't going to lower her BP with dark chocolate and green tea vs light chocolate and coffee.