Darkness is dark.
Torches and the Light spell provide bright light for 20 feet, and then dim light for another 20 feet.
A dwarf (with 60 foot Darkvision) standing next to a lit torch could see for 20 feet as if in bright light, 20 feet beyond that as if in dim light, and then, due to Darkvision, a further 20 feet as if in dim light but in grayscale. (Unlike earlier editions, having darkvision nullified by nearby light is not in 5e.)
But how far away can that light itself be seen in the darkness? If there's a huge underground room 200 feet long, and our torch and dwarf are at one end, can a human in darkness at the other end of the room see the light and dwarf? Does the human still count as blinded because he is currently in darkness?
Best Answer
You can see light at any range
I did some googling, and while it's pretty hard to find specific citations from scientific studies, the places that I've been able to find say that the human eye can see a candle from somewhere between 10 and 30 miles away. The curve of the Earth is about 3 miles away. Thus, any significant light is at least barely visible from any range that you're likely to have line of effect. The houserule that I've used for a while now is that you can see a light source at ten times the distance that you can see a non-lit object without penalty.
That said, the vision rules in every edition of D&D that I've seen are actually reversed. They only work if both the looker and the looked-at are in the same lighting conditions. The vision rules on PHB 183 state:
Nothing in those rules says that you can't see a creature if that creature is inside darkness and you aren't. This is clearly ridiculous. This weirdly reversed rule has existed since at least 3.0, and shows how little the designers thought about what to do about differing light conditions.
What this means is that you should rely on your intuition more than the rules for what will give penalties based on vision. Since, IRL, lights can be seen from the horizon, you can probably see someone with a torch from at least a few hundred feet, probably out to a mile or so.
As far as being blinded is concerned, my intuition was always that that penalty happened because you couldn't see the ground beneath your feet, or the things that are around you. Thus, I would rule that a character who can see a light hundreds of feet off is still blinded, except for the purposes of making checks or attacks against targets who are lit up.