I don't like to add blog settings in site settings because blog is not a core feature, it is shipped with mojoPortal but is part of the mojoPortal.Features set of projects. The core has no dependency on features, features depend on the core. Html and Newsletter are both part of the core projects. The main reason we have a separate setting for Newsletter is because html for email is so different than html for web content and it requires the full html page markup whereas content features only edit html fragments.
Note that you can set it to TinyMCE as the editor in Site Settings then in the settings for Html Content we have a setting to force a plain text area. The only down side is you have to enable that setting per instance of Html content.
Hope that helps,