When we initially launched seven.centos.org, the idea was just to have a single blog instance that CentOS Dev and QA team members could use to give feedback and also report status update about the rebuild and testing of CentOS 7 : that was an easy entry point for people wanting to know how far we were in the process, what to expect, etc (and so give more transparency that during the CentOS 6 rebuild era) ... That was in 2014.
Then it continued to be used by some contributors who wanted to give hints or talk about CentOS 7 new features, but without having a personal blog (or if their personal blog wasn't aggregated through our http://planet.centos.org instance). As more and more people joined the CentOS SIGs , seven.centos.org was more and more used a central blogging platform around the CentOS ecosystem, and so not really anymore about the status of CentOS 7 itself (which was released in July 2014). We even linked authentication against our (deployed in the mean time) https://accounts.centos.org (through OpenID).
So we thought it was time to rename it to blog.centos.org, to reflect the reality. All previous links/permalinks are still working, but default URL is now blog.centos.org.
Happy blogging !