This issue appears when a PNG is saved with indexed color in CS5 and below.
Photoshop cannot properly display PNG files that have indexed color for transparency because of the way the transparency data is embedded in the alpha palette versus being stored in a separate alpha mask.
The result is Photoshop reading the RGB components of each indexed color and ignoring the the transparency data embedded in the alpha palette of the image. In a normally-saved PNG, this data would be included in a separate alpha mask and be read appropriately by the program. In this case, with the transparency data unable to be read, the image background turns black.
You can read more about this here: Why can't Photoshop properly open this PNG?
Later versions of Photoshop are able to display indexed color PNGs with their intended transparency.