In Blog.execute(), line 91, you call the xmlrpc proxy with args as an array:

        r = getattr(self.server, methodname)(args)

This causes the method to be called with a single <array>param1, param2, ...</array> parameter containing the individual parameters rather than the individual parameters directly (<param>param1</param>, <param>param2</param>, ...).

This call should be:

        r = getattr(self.server, methodname)(*args)

so the underlying xmlrpc call uses the parameters exactly as passed.

Interestingly, I found that when calling the WordPress API, WordPress handles the call correctly either way. Internally, it must see there is a single array parameter and unwrap it. However, when calling a different metaweblogs service, the parameters were not interpreted correctly as an array. I checked the spec, and the non-array version is correct as far as I can tell.