I objected politely to a comment a politician made on their Facebook page and they deleted it and blocked me.
The answer depends in part on what type of Facebook account you were blocked from. While politicians are public figures, they are still entitled to control their personal Facebook accounts the same way that any private citizen may, choosing to block or filter whomever they choose. If, on the other hand, the account is designated as a particular agency account or as representing the office of the politician, the issue is more complex.
The question of whether a government agent deleting a comment from Facebook or other social networking sites counts as a First Amendment rights violations is an unclear issue at this time. It's another case of technology outrunning the law. First Amendment cases from the Supreme Court and other leading authorities mostly focus on physical public forums, such as protests in parks or public universities. As a result, it is hard to predict how a court would rule on a case where the supposed forum is a social media site (or other online "location"). There are many unsettled questions around online forums, such as what kind of public forum they are, what intent is required to create an online public forum, how the government may restrict users' posts, and so on.
I wish I could give a more definite answer, but I believe time will tell how much protection is afforded to social media sites. I'm glad you raised this topic because it's a relevant problem in need of an answer, and it looks like others feel the same way. See the link below for an ongoing case that seems similar to yours. It will be interesting to see what the final result is.