I'm not sure that a block chain is required at all
What prevents double-transfer of names, then?
but if it is, it's probably best to use the Bitcoin block chain.
That is dangerous because of the developers'
active hostility towards attempts to store non-transaction data in the chain.
it should be allowed (with appropriate fees), and in practice it's probably difficult to prevent it.
Unfortunately it's very easy to prevent and
has already been discussed on bitcoin-dev. It can be implemented the same way P2SH was added: a miners-only upgrade that is forced on the minority once the majority adopts it.
You might wake up one day to find that the currency you'd been using to store key/value pairs is no longer accepting updates.