The Bitcoin network operates over TCP sockets between nodes. This has several consequences.
- Blocking URL's or domain names is not possible.
- By default, Bitcoin uses port 8333. It's possible for a school/uni to block this port. For an ISP, however, this is not possible, other applications might use port 8333 as well. Also keep in mind that it is very easy to change the port your Bitcoin client is using.
- All Bitcoin packets start with the same "magic byte". This means that it might be possible to inspect all TCP packets looking for this magic byte. However, this might break many other internet functionalities, so I doubt if this type of blocking would ever be performed.
Important to note is that this two ways of blocking can easily be dodged by using an encrypted connection, f.e. with a VPN.
So basically, it is really infeasible for any party to block Bitcoin traffic because all possible blocking methods can easily be avoided. This holds for most P2P technologies, look at BitTorrent f.e.