How can I black out sunlight BEHIND my condo's windows and vertical blinds?
I agree those window coverings are wholly inadequate for keeping out light. They're the &quot;cheap, default&quot; window coverings provided by most condo associations.   You're expected to upgrade if you own the place.
Normal upgrades include heavier curtains which do block the light.  Note that most quality curtains have a &quot;curtain liner&quot; on the backside.  These are typically white or off-white, even if your side of the curtain is red, blue, black or whatever.   It might even be a variation on the blinds you do have, with the interior side black instead of white.
So they certainly don't mean you can't have window treatments that are non-white on the interior side.  They mean if you're going to put up trailer-trash third world literally garbage up to block light, you need to stand it off from the proper window treatments so you don't mess up their aesthetic.
You live there because  you like it, because it's nice, right?  Well, it didn't get nice by itself.  It's nice because the builder, and the condo association, and the residents, work pretty hard to keep it nice.
&quot;Nice&quot; has other implications, like higher class occupants and less crime.
If you really mean to say &quot;nice&quot; is not important to you, then you're wasting capital by tying it up in a property whose major (costly) feature is being &quot;nice&quot;.  Sell it and get something that's a better fit.