The blog may be physically hosted at two locations, but the domain name will only be pointing to one of the two at any given time. That's how domain name servers work. They communicate with the registrars to route traffic from people's web browsers to the correct servers that the sites are hosted on. So if your DNS settings were posting to your hosting account on Bluehost, and you switch over to GoDaddy.com, once you get your site set up there you just tell the DNS settings to point to them instead.