You can darken the headlights but that's a LOT of effort. Blindfolds are much easier.
Seriously: if you're after a look like Jased22 pointed out above, you might find that buying them is easier. Also as pointed out, actually making the glass darker (which directly equates to less light being transmitted) will make you a target for cops. They might not be after you today, but they might one day, or maybe one will have a bad day and pull you over and give you a canary just to get back at the world.
