The main advantage of national coins, it seems, is that you don't have a bunch of random people all over the world minting the coins and dumping them, instead the nation can issue the coins in return for things of value and use those things of value as "reserves" with which to "back" their coin.
See for example Canadian Digital Notes (CDN), United Kingdom Britcoin (UKB), Martian BotCoin (MBC) and United Nations Scrip (UNS), tables and plots are online at
http://galaxies.mygamesonline.org/digitalisassets.htmlThe disadvantage of course is that using a blockchain is insanely expensive, basically it is not practical, because you are not likely to get many miners, even using merged mining, until the transaction volume is huge enough to make the transaction fees add up to enough to attract miners.
So unless your nation has massive massive mining farms of its own with which to secure its coin, it is likely that using a blockchain is simply not going to be feasible; some other method of securing their ledger will be needed until such time as the volume of transactions gets big enough that the transaction fees start to look like enough to be able to secure a blockchain...
The advantage seems to out-weigh the disadvantage though; as those tables at the link above should make clear.
Originally it was expected that bitcoins would be worth far more than any of the "national" coins; however the fact that anyone anywhere can mint bitcoins and dump them seems to have resulted in bitcoin value constantly being suppressed by "dumpers", so that long long ago now the "national" coins far surpassed bitcoin.
-MarkM-
