In certain parts of the EU there are restrictions on merchants accepting cash for transactions over a certain amount.
It was my understanding (perhaps incorrect?) that these laws prohibited even private party cash transactions, not just merchant transactions.
Assuming correct (if not, wait a little while and it probably will be), this same law could likely apply to anonymous virtual coin transactions between private parties.
Anyway, at the moment I don't see much of a movement toward banning virtual currencies, and this is indeed somewhat surprising relative to expectations a few years ago, although I think the winds could shift on that, especially with respect to untraceable virtual currencies. Nothing is certain though.
Here is an example of the law in Spain. It bans bearer (not just cash) transactions that are payments from consumers to businesses, so it would also apply to crypto currency. Spain in a country where tax evasion is the national sport, followed by football (soccer).
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My take, knowing the culture well, is that this is doomed to failure since people with just use the "black money" for many day to day expenses that fall well below the threshold.