When sending HTML email a text version is required. Did you enter something in both the text and HTML sections?
For the text section it may be more convenient to make use of a message code of the type "Link to HTML", which will be replaced with a link the user can click to view the (personalized) HTML email in their web browser.
If you simply enter "test" as the subject, text message and HTML message and it still comes out blank, there must be something else at work. In that case I would be willing to investigate hands-on with access you can submit securely
here.
Regards