Since your image appears to be correct except for the background color, the image most likely not corrupted.
A black background in the transparent regions of a PNG are most often caused by the viewing application you use, or by the application used to export the image to disk. The application is apparently not capable of displaying the transparency correctly.
As PNG pixels consists of four color values (Red, Green, Blue and Alpha, RGBA), any viewer or editing application that ignores the alpha component will just display the remaining color components. If those RGB components are all set to 0, the resulting color is black.