I was wondering if there is a single API documentation with “argument list” and the detailed description (and struct definition) of all the return values for all the blogging APIs like Blogger API,mt, metaweblog, and wp?
In-depth Documentation of XML-RPC API
As far as I know, WordPress doesn’t have its own API, it simply supports others. The WordPress Codex has a section on its XML-RPC support. It says there that they support the Blogger API, metaWeblog API and the Movable Type API.