Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
We tried a couple of programs on your image and they all concur your image does not have an alpha channel. Which program are you using that suggests it does have an alpha channel?
We're using ImageMagick 6.2.9-6, the current release. We get reasonable images without artifacts. Try saving the results to PNG and viewing the image with a web browser. Do you still get black areas in the corner?