A C-block refers to the third set of numbers in an IP address. Example: 126.96.36.199. Each website on the Web is actually an IP address and the domain name (like example.com) routes your computer to that IP address.
Hosting providers buy large blocks of IPs, so if you have a lot of websites hosted by the same provider they may be in the same c-block.
One tactic that some people have used is buying many domain names and making websites that all link to a target website that they're trying to boost the SEO and search rankings for.