I don't have a space blanket handy at the moment, but I have one of those mylar bags that electronics are packaged in. Yup, you're right. Pointing it up at the sun, light does get through.

Thank goodness, I can still keep my mylar-lined hats around to keep the guys in the black helicopters from stealing my thoughts. Wait, maybe I've been wrong about it actually being good even for that purpose...

I'll put this part first and then repeat it. DO NOT use a space blanket for this, but it is an interesting fact.
Mylar filters are one of the few materials recommended for viewing solar eclipses. WARNING!! - The mylar used for solar viewing is a special version. Space blankets are NOT suitable for this. (Unless you like having your retinas burned...)
-phil

For solar stills, you'd want a light gauge, clear plastic. Thick black plastic would be the least effective.

They were set up lean to fashion with a hole underneath to record
temperature. There needed to be at least 3 feet from the thermometer or the heat absorbed by the plastic would radiate to
it. It was so hot that we had to wear boots with thick soles and
wool socks or your feet would burn from the ground.

The coolest places were natural shade spots, as they handn't been
heated up prior, and were the most opaque of all.

Yes, clear plastic makes a better solar still, but black works too.
A friend of mine traveling in the Outback of Australia would tie
black plastic garbage bags around vegetation to collect the water
that would cook out.

Clearwater, did you test multiple sheets? At some point, I'm sure it's overkill, but if one sheet of black plastic is good, it seems that two or three layers would be better?

I wonder if two sheets with an air gap would be better than a single sheet, or simply stacking two sheets together? Top sheet heats up and heats the air between the sheets. That pocket of hot air rises and drifts up and away by convection. Also, the inner sheet may help block the heat that gets through the top sheet. Anyway, just a theory.

In the US army survival manual it recommends what you describe. Double layering the tarp material for a desert shelter with an open space (about 12') in between the layers. Supposedly it can reduce internal temps by 20-30 degF. Haven't tried it yet but I see no reason why it wouldn't work.