Do black out curtains work during the day?

Shortly I'm switching my photoperiod seedlings from 24/0 to 18/6 for vegetative and I have to run dark during the day because of heat. I'm worried about the light coming in from a vent that I have to leave open in my grow tent. Am I wasting my time with black out curtains? I already have blinds on the windows but they don't do much when it's sunny. I'd like to have something figured out before flower and I only plan on vegging for 2-3 weeks.

Any other suggestions? Thanks!

I have also been wondering that, although perhaps you can just drape the black out curtains over the window

Not sure on the black out curtains but you can light proof your intake vent in the tent and not worry about the room being pitch black. You'll probably find threads on light proofing your vent right here on RIU.

the black out curtains will work but you will need to make a tight seal around the widow, other wise light will get in from the sides. Don't get the 99% ones, 1% turns out to be a lot more than you think.