Sorry for the confusion people. Quintus works like this: if you add a header image but don't explicitly set a header text color, then it wraps the title and description with the default black textured background. Otherwise, you would probably end up with legibility issues by default.
So, like Panos discovered, to remove the black background you need to explicitly set a color — that means you are choosing the desired color contrast between title and image for your blog, so all is good. :)
It also adds another possible layer of customization. If you still want a white blog title you could put a #fefefe color.
Hope it makes more sense now.
@jazzverbatim it seems you solved the background issue by now. Anyway, ff you change the color or add an image in Appearance → Background it will overwrite the default background. Clearing the field restores it.