I'm a bit confused about the light for my turtle aquarium because I've read so many different things. Is it okay to leave the turtle in darkness during the night?

Yes, the turtle should be in the dark at night.

should we shut off the light every night?!

Yes, the turtle should be mostly in the dark every night. My turtle has purple incandescent night heat lamps. They don't create any white light but do create enough black light to see a little. Basically, if there's too much light for you to easily sleep if you were in the turtle's tank, then it's too much for night time for the turtle.

Best thing to do is buy a timer and set lights on a timer, it really is a big help