I personally think China won't resort to blocking foreign exchanges (via their great firewall), I believe if it's something they intend to do, they would have done it by now - if anything, it would have been one of the first things that they would've done (and I'd say they would give their citizens a chance to take their crypto's out of there before immediately banning foreign exchanges). I also don't see any reason why they would unless they're going to make it illegal for a Chinese citizen to buy Crypto with CNY (which they haven't as far as I know). We have to remember, the bans are currently on Chinese ICO's and Chinese Exchanges. Yes, there is a document circulating about them banning mining and foreign exchanges (in the same document), but it's most likely a fake (whether it could happen or not is a different question).

Putting aside of what they plan on doing ... let's assume they don't ban foreign exchanges (yet, or ever). Is there any reason of why a Chinese citizen wouldn't be able to buy crypto's from a foreign exchange (USD/EUR/GBP...etc)? Again, I'm not really sure how Chinese banks work when purchasing anything online in a different currency, I assume it's no different to the UK where if I pay for something in USD, the bank will just convert it using their rates and charge a foreign transaction fee.