While I can sympathize with your concern that political entities such as Russia and China are 'experimenting' with blockchain, any implementation of cryptocurrency that wasn't decentralized, cryptic, and delivered of value by the valuers thereof, to wit, the possessors of the currency, would quickly be ignored to the extent possible by users of currencies possessed of those features.
Also, China and Russia face currency manipulation on a scale that may be difficult to conceive, and it may be that both are interested in currency that is manipulation proof. And counterfeiting proof. And theft proof. And I wouldn't blame them.
So, there is yet hope, and no need to despair. If they try to make another kind of funny money, just one that isn't printed on pieces of paper, they will reap what they sow. People will seek and find the cryptocurrency that best meets their needs.
From Golem to Steem, cryptos can be designed to meet those needs. If these economies do adopt actual cryptos that meets the needs of people competitively, whether proprietary or something extant, this will not negatively impact cryptos, but pump huge caps into the crypto corpus.
Even if that only trickles into extant cryptos, that will still be a good thing for holders of those currencies.
tldr; If the BRICS make crappy crypto, it won't be worse than the situation now. If they make or use good money, everybody wins. How can they take over extant blockchains? If they can take over one, the next one will find a way to prevent it. Why should they bother?