first of all since not all the government of the whole world are going to do something like this, bitcoin won't be blocked. in other words there are currently so many countries that have adopted bitcoin with open arms and legalized it that even if some governments decide to block bitcoin it won't matter.
However what would happen if a government decision to implement DPI blocking of Bitcoin protocol?
DPI is only when your packets are encrypted such as when you use VPNs. bitcoin uses no encryption for communication. it is all performed over simple HTTP protocols.
and there are things they can block but they can not block bitcoin usage completely, things like that would only make running a node harder not impossible. and they don't do anything to bitcoin usage. you can always sign your transactions offline and broadcast them by opening a website like this
https://blockchain.info/pushtx with a public SSL proxy and pushing it through their service!!! free and easy.
On top of it exchange could be blocked with a firewall on top of all others kinds of bitcoin forum etc...
exchanges are third parties that can be blocked, yes. but it would be like blocking a website. you can always use a VPN to access them!!!