When I ride on a bike path alongside an unlit road in the dark, I find myself blinded by the lights of oncoming cars, even when their lights are dimmed. This is even more true if the bike path is on the left side of the road. Currently, I mitigate this by holding one hand in front of the lights of the oncoming cars, but I wonder if there are superior strategies. The military tend to use bright flashing lights as one weapon to blind their adversaries, so I expect they have also developed defensive strategies against such blinding. How can I prevent being blinded by the (dimmed!) lights of opposing cars?
Also, you can avoid looking directly at the headlights. Look slight off to the side, and maybe downwards a bit. I'd probably look at the edge of the path the farthest from the road. That way, you'll see things on the path, without your eyes looking directly into the lights of the car.
You'll still be able to see the path just fine.
Use a different route, if one is available. This is obvious but I think it is actually the best solution, if it applies.
If you wear glasses, make sure they're very clean. If it's raining, consider using a different route even if you wouldn't in the dry..
I find that a cycling cap under my helmet helps quite a bit if I tilt my head down and towards the traffic so the peak is at an angle. It blocks off some of the road while not blocking the view forward too much.
I find it helps to run my front light brighter than I normally do, when I'm using such a cycle path. Then there's slightly less contrast between the path infront of you and the oncoming lights. Be prepared to dim your light if somebody's cycling the other way and make sure you're not being obnoxious to the motor traffic.
Ultimately, there doesn't seem to be a better shield than your hand.
Maybe if you use a helmet with a sunshade (I have one), you can tilt your head downwards and sideways to block the car lights while still seeing forward a safe distance.
By the way, I detached the sunshade on my helmet because it rotated too much forward/downward with rucksack and thick clothes in the winter and it cut too much forward visibility.