Would it be possible to provide a downloadable blockchain that is updated and verified by a mass of people?
There already are downloadable versions of the blockchain. There's no problem in principle in having them GPG-signed by several trusted people. However:


It somewhat weakens security. With the software itself you can trust the signatures and you can inspect the code. With blockchain data inspecting it is infeasible.
Future versions will use a different DB format which allows for much faster verification, so there will not be much advantage for a system where verification is obviated but downloading the data is still needed.
In the long-term, end users will not use a full client, but rather (at best) an SPV client which does not need to download or verify the entire blockchain.