This is a placeholder issue right now for a core API change from #204497: Make the background fill color configurable where image styles result in blank space. image_imageapi_imagemagick_crop() (from imageapi_imagemagick.module) needs rework to take also the background as parameter.

After fixing #204497: Make the background fill color configurable where image styles result in blank space in 8.x, I will provide a patch for imageapi_imagemagick.module D7.