Sunblock is not supposed to reflect heat (infrared). It's supposed to reflect UV. Different spectrum. Kind of the whole thing the OP was about. I have no idea if sunblock also reflects infrared though, incidentally.

https://care360.basf.com/emea/en/industries/personal-care/co...

Sunblock mostly absorbs UV (and turns it into heat). Some reflection occurs but it's mostly absorption.

After looking into this even more, there are two types:

Mineral - Reflects

Chemical - Absorbs

I thought so too but it's not the case. Even the mineral sunscreens mostly absorb.

https://cen.acs.org/business/consumer-products/What-in-sunsc...

Video with a UV camera is interesting: https://www.youtube.com/watch?v=E9tEEDpeJoo

Black means absorbed, of course.

And, omg, you can buy an active version on amazon, for this exact purpose: sunblock coverage check!

https://www.amazon.com/s?k=uv+mirror+for+sunscreen+test