Reasonable use of Custom Content Types?
I would not expect any performance problems on a site with 30 different content types. Nevertheless I wouldn't recommend implementing a scheme like you are intending. Should you come into the situation where you have to add a new field or change field/display settings, it will require you to go over all blog-types.

In order to simplify the creation of new content for your authors, I recommend to create shortcut links where the term-id of the blog is passed by an url parameter. You may use the Prepopulate or the Entityreference Prepopulate module, or simply alter the node_form in a custom module.