Posting in the Magento forums has been disabled pending the implementation of a new and improved forum solution which should better serve the community.
For new questions please post at magento.stackexchange.com, the community-run support site for the Magento community. We will be providing updates on the new forum solution soon. For questions or concerns please email firstname.lastname@example.org.
There is no need for a background color. All the thumbnail functions would do is resize your image, so whatever is in your image is what you get. Nothing added.
So if you want black, make your image black.