Window covering?

[Bob's Auto's]

Orca grow film for the win.Light does not penetrate .

And if you want white on both sides you double layer the film and voila...

 

[BII]

Heavier black plastic garbage or yard waste bags taped together, cut to size and hung by a few pieces of tape will work too and cost nearly nothing.

But if it's a very visible room with lots of window space, go for some type of curtain (or blinds) being visible to the outside. Most of the other options cited, such as films/sheets of plastic, will look out of place, not normal for a sun room.