Ah, I was to quick in asking, without doing proper research. No, it is not possible (currently) to run many blogs under many domains, served by one installation of Ghost.
To really serve many blogs, you need to have many installations of Ghost bind together with for example Nginx or any other solution for redirecting traffic from each domain to each specific installation of Ghost.
More information can be found here:
Ghost has introduced a feature of having many users in one Ghost blog, but as for base implementation of Ghost, there is absolutely no support for running multiple blogs / domain under one Ghost installation.