Google+ recently came out with an update to Auto Enhance your images. It's set by default and can go unnoticed until you need to upload a transparent .PNG image! These images will show a black background thanks to this new feature.
Any images you upload to Blogger in Post or the Header will show a black background unless you uncheck the settings.
- Go to your google+ settings http://www.google.com/settings/plus
- Find the Photos section and uncheck Auto Enhance!
Don't have or want a G+ account?
Upload your images to TinyPic or Photobucket.