The unknown reason
all but a few persons are rendered blind, is that they had watched a fireworks
show that circled the globe. Only the fireworks were what we have come to know as orbital atomic weapons.
Some sort of accident, of course, cascaded from one country's satellite to the next, initiated the colorful display. And everyone got everyone they could out to watch--and were blind the next day. (The narrator had somehow injured his eyes, and they were bond during the lightshow. There was noone to unwrap him, so he knew that something had happenned.)
And the triffids were a kind of animal-plant that produced an oil that was extremely useful. But they also had teeth, or clawlike appendages that were poisonous, or could kill. When they were removed, the oil was of a lesser quality, so, plantations of them were cultivated with the claws.
It was a great novel.