When exporting images with additive blending in Spine, the result can appear black if the background is transparent. This is because additive blending relies on the background color to produce the desired effect, and with a transparent background, the blending doesn't work as expected. To resolve this, you can either avoid using additive blending for static image exports or ensure that your images use transparency instead of black. Additionally, when exporting, make sure that your Spine version is up-to-date, as older versions had issues with additive blending during export.

Thora As Spinebot pointed out, I thought you were using the Additive blend mode, but is that not your case? Exporting when using the Additive blend mode is described in the following section of the user guide:
https://esotericsoftware.com/spine-export#Additive-blending

When a slot uses additive blending, its attachment is rendered additively when the image or video has a background color. If the background is transparent, the attachment is rendered additively when over other images, but it is not rendered additively where it is over the transparent background. If the attachment image uses opaque black, the black will be seen where it is over the transparent background. To avoid this, use transparent instead of opaque black.