That's a common misconception. The main barrier was that the soviet currency wasn't exchangeable and possession of the types that were (so e.g. USD) was illegal without official permission.

There was a chain of stores in the Soviet Union named Bierozka with western goods you could buy with USD - officially for tourists and workers from abroad.

Of course there was a flourishing currency exchange black market, but it was tolerated because it enabled the citizens to get rid of their foreign currency - something which the state wanted.

It's not true. There were official import channels. Not many consumer goods were imported but some were. Secondly, there were shops where travelers (sailors, pilots, etc) could sell the goods they bought abroad. Thirdly, soviet people worked abroad, e.g. building plants, factories, power stations. These people were paid in special kinds of checks and they could buy foreign goods in a network of special shops (Berezka). I know this because friends of my mom worked on building some powerplant in Egypt (I don't remember exactly, but it was Middle East I think) and bought many things with those checks.

The whole USSR existed thanks to free black market that was unoficially allowed (yet somehow controlled) by the state.

But there was illegal access...