Click import with a blueprint opens the import blueprint window only.
export blueprint string and importing this with the window does also not work. as expected.

I assume the blueprints are used directly from the blueprint library. That's a known issue, the API currently doesn't let me read all necessary information from blueprints/books in the library. Until i create a workaround you'll have to create a copy of the blueprint and use that to import.

OK, I know this issue. Maybe in the meantime you should show a message that such blueprint can not be read.
Do you know if anyone has made a request to supplement the API?