Hi Will,
A C-block is part of the IP address of a website (xxx.xxx.CCC.xxx), for example, with my blog, the IP address is 173.248.188.141 - the c-block is 188.
Usually, websites that are all hosted on the same server have the same c-block. This is a way that Google can detect link-farms, where loads of sites are set up on the same server and used to link to your website. This is why it is good to have diversity in the amount of linking c-blocks.
Inevitably, you will have some identical c-block links but that's ok (and is completely natural). In terms of the links to your website, you have links from 130 root domains that are made up of 70 unique c-blocks. This ratio is perfectly fine, so don't worry.
Hope this helps.