I wonder if having a blockchain being digitally signed by a central authority (scrapping out the 51% requirement) would create something like "cryptofiat".
I failt to see why they would do this, but this is secondary. If a governement cannot intervene on transactions, I think it would not qualfy as cryptofiat.
On a related note, I suggest reading
The Stateless Currency and the State: An Examination of the Feasibility of a State Attack on Bitcoin. It is damn interesting.
Bitcoin, therefore, implicitly relies on the state’s approval for its functioning.
And also
State-sponsored cryptocurrencieshttp://www.bankofengland.co.uk/publications/Documents/quarterlybulletin/2014/qb14q3digitalcurrenciesbitcoin1.pdf, page 3
As emphasised by Haldane and Qvigstad (2014), it would technically be possible for an existing central bank to issue digital-only liabilities in a distributed-ledger payment system equivalent to those deployed by recent digital currencies.
This is the Bank of England which writes this