Which roller blinds are best to keep heat out?

The very best will be the ones that are outside the window.

No joke.  The best place to stop heat from getting into your home is before it ever gets in at all.   Before air conditioning was a thing, homes had awnings over the windows to stop heat before it ever even hit the window.

Barring that, your next best bet is going to be the most reflective blinds you can find.  The very brightest white if you want a bit of light to still filter through, or if outside light isn't important then a blind with a mirror-like aluminum surface would be good - aluminum reflects infrared heat very well, even shortwave IR like from the sun.  Shortwave IR goes right through glass, but glass tends to hold longwave IR - like the heat re-radiated from objects that are now warmed up - which is why it's so important to stop the heat before it gets in.

Also consider some of the nanoceramic window films that are available.  Some of these are water clear and don't affect the view at all but are quite good at reflecting infrared heat.  These are available for installation on cars but call some of the installers that do cars and they may be able to hook you up with a residential installation.

Good luck with it.