I'm not sure what you mean by "default phone image viewer". Do you mean Android's standard "Gallery" app? If it is displaying a black image, well then that is a bug on Google's end, not ours. The photo that you are saving to your Documents directory is just a "file copy" of the image file in the Gallery app.
Now, if the image that is in your Documents directory is solid black, then that's a different story. If that is the issue, then what happens if you display that image via
display.newImageRect()within Corona? Does it display correctly? It has for all of my tests. I've also successfully e-mail that saved photos via our
native.showPopup()API as well.