Curtain (night & day ). I feel complete dark room is very conducive for night slp & can use day curtain for naps. I have blinds and the strings are a strangling hazard when they are older. And with blinds, lights still come in. Can never achieve total dark room which I regret . And If wind blow (fan or natural wind), the blinds will sway & hit the wall & produce noise which might wake baby.
For newborns we should try to keep day time bright and noisy and night time quiet and dark. That way they can slowly adapt to day-night cycle and will learn to sleep more at night and less during the day. So it's not necessary to totally block out the light, but if you have direct sunlight shining in it's better to cover up because better not let the sun shine directly on baby
Maybe blinds would be better so you can control how much sunlight you want in the room? That's an option 😀