The gist of it is this:
PREMULTIPLIED ALPHA BLENDING is a fast, standard transparency blending algorithm for game graphics. But it requires a specific math operation (a multiplication step per color) to be done ahead of time.
Saving images with "premultiplied alpha" means you're preparing an image to be blended using the above algorithm. This results in an image that gets misinterpreted by most image viewers (including TexturePacker for some reason) as having a black border. But more importantly, that doesn't matter. When it's set to be interpreted correctly in the game engine, it looks ABSOLUTELY CLEAN.
From experience in Unity, working with Premultiplied alpha is both faster and cleaner (as in no weird white, black or gray artifacts at the edges which happens often in some other sprite/skeleton systems' atlas tools, and sometimes when fresh out of Photoshop too) compared to regular alpha blending.
That said, taking full advantage of the premultiplied alpha blending algorithm requires two things:
(1) Save your image with premultiplied alpha channels. (check the checkbox that says premultiply alpha)
(2) Tell the graphics processor to use premultiplied alpha blending. (in this case, using the Skeleton shader that comes with Spine-Unity in your material asset)
