Does the amount of light Blackout Curtains allow in reflect on its heat reflecting capabilities?
Dark curtains will reflect far less heat back to the outside, and thus heat up in the sun and radiate that heat into the house.
So it's not just infra-red light (a.k.a. heat light) and visible light passing through the curtain that heats the room, it's also how much infra-red light is absorbed that matters, as it raises the temperature of the material.
Unless the curtain is made of a darkening AND heat reflective material (e.g. thick curtain, facing white towards the outside), it's going to be a lot hotter inside.