Please open your eyes and see the reality, which is that textures are filtered and all channels are interpolated along the pixels. That's the way it is done in every modern graphics engine. Your assumption is wrong because it does make a difference, but it seems like you'll never agree. As I said I'll have to use both Paint.Net and Gimp to accomplish this then. I remember reading on the official png page how they described Photoshop's handling of the matter some time ago as plainly wrong. (It's like in Paint.Net, only that they used to set the color to white). Note that fixing this would require very little effort from your side, since it would merely be a removal of a "feature" which defaults the colors. BTW, why are you using black and not white or pink? I remember pink being a designated color for transparency in many old DOS games. I personally like green too.
Let me state this very important fact once again: While filtering textures (e.g. bilinear, trilinear), it does make a difference what the color channels' values are when the image is 100% transparent in places. Your method causes uncontrollable black outlines. Also please note that some shader systems give you the option to just ignore the alpha layer. An example would be a green texture with jungle bushes which could optionally have transparent holes between the leaves and also be set to be 100% opaque but still look nice.
Why limit the user in this way? If you don't automatically set 100% transparent pixels to black, this can still be done, while the current implementation imposes an unnecessary limitation. You could add a little dialog for setting 100% transparent pixels' color values to something, but as a real feature and not a silent limitation. I'd always yield to a solution which allows for more things, and that would clearly be not defaulting those pixels.
See my font in action:
http://www.intercomm.com/shrinker/images/08/font.tech.test.png
Using only Paint.Net, it'd have ugly black outlines. Zooming out, mipmaps would make it look gray.
Also note that during mipmap calculation, your black color makes subsequent mipmaps darker and darker because of interpolation effects too.
Such a generic gaming scene has a lot of transparency going on too, and as you see, texture filtering is a must:
http://www.happypenguin.org/images/doom3.jpg
Recommended reading:
http://en.wikipedia.org/wiki/Texture_filtering
http://en.wikipedia.org/wiki/Magic_Pink
http://en.wikipedia.org/wiki/Mipmap