One road with that I would say has the most blind summits is the A68 between the A66 and Carter Bar on the border. That section of road goes over valleys rather than along them.
My interpretation of a blind summit would be where the traffic travelling at normal speed (well within the speed limit) towards each other will not see each other until the last second, which is why they are dangerous and marked with hazard, or preferably no overtaking double white lines.
There was a recent and long term advertising campaign about a motor cyclists and various road conditions. Markings on roads plus signage indicated 'humps and hollows' especially when trying to overtake ;o)
"Blind Summit" signs in this neck of the woods usually mean, conversely, a dip in the road which could hide an oncoming car even though the road looks clear, i.e. you are on the summit, but you have a blind spot ahead.