Is blocking certain websites a form of censorship?

There's a difference. If government imposes it upon the Web, then yes. If I install it on my own personal computer to block sites I personally find objectionable and decide is inappropriate content for my own family, then this is simply a form of personal liberty and personal choice. It is not being imposed upon me by any outside force or influence. You have the right to speak, and I have the right to ignore you.