I should probably clear the documentation and examples up, and be a lot more specific.
If I was developer asking for text/vcard, I would expect either an single instance of vcard data or URI reference to the data, or an array of vcard instances or URI references.
The instance could be anything that is Transferable or Cloneable using structured clone so: Object, Blob, data uri or string. Currently it is on the burden of the developer to parse and check the data, and to check if it is a single isntance or an array.
I will clear up pick to say that it will not take data, but the type is what is expected in return - it is just that the representation of the type might take multiple guises. It can't use be a URI because we need this to work offline - so a blob or JS object can work well in these scenarios.
Developer Advocate @ Google for Chrome and HTML5