If the 3D model you've uploaded to MyWebAR appears black, please make sure that you have at least one light source on the scene. If you're not sure how to set up light sources, you can simply create a new project and it will include one directional and one ambient light source by default.
If you have active light sources on the scene but your model still appears black, this might be caused by compatibility issues or missing textures.
.glb is the most compatible and optimal file format for webAR. It allows to avoid compatibility issues, as it bundles all textures, materials, mesh data and animation tracks into a single file.
If your model doesn't have any issues in the 3D software but appears black in MyWebAR, please try to export it directly into the .glbformat to see if it helps.