You are actually asking two questions here. One is about *censorship resistance* (can somebody block the bootstrapping process?) and the other about *authentication* (can a bad person pretend to be a Tor relay?). Here are answers to both questions:

 1. **Censorship resistance**: Yes, a nasty network administrator can indeed block the bootstrapping connections which download the network consensus from the directory authorities. In fact, this is done by the [Great Firewall of China][1]. (Part of) The solution to this problem are bridges. [Bridges][2] are basically unpublished Tor relays which no longer have this single point of failure. Additional information can be found under point 'h' [here][3].
 2. **Authentication**: As you write, the Tor source code includes a hard-coded list of the directory authorities' IP addresses as well as their cryptographic key fingerprints. This is the *trust anchor*. After your Tor client fetched the network consensus which contains all Tor relays, it verifies that it is authentic by checking the signature over the consensus. Nobody can give you a manipulated consensus because your client already knows the authorities' keys. The consensus then contains the key fingerprints of all Tor relays which makes it very hard to "man-in-the-middle" such a connection. Of course, this assumes that you got your copy of Tor over a *secure channel*. In practice, you probably downloaded it over HTTPS from https://www.torproject.org and checked the PGP signature. Unfortunately, secure software distribution is a difficult problem. Additional information is available [here][4].


  [1]: http://www.cs.kau.se/philwint/static/gfc/
  [2]: https://www.torproject.org/docs/bridges
  [3]: https://www.torproject.org/docs/documentation#UpToSpeed
  [4]: https://www.torproject.org/docs/faq#KeyManagement