It's not a rendering problem. The skeleton shader just requires a specific, but standard, transparency format. You need to run your images through TexturePacker and make sure "Premultiply Alpha" is checked. (this makes it faster to render. It'll look weird in most image viewers but it'll render correctly in-game.)
Or, you can modify the shader Blend formula.
Nate put the additive blending checkbox in Spine editor and the data is included when you export to JSON, but the runtimes don't support it yet.