This is a placeholder issue right now for a core API change from #204497: Configurable background color when cropping to larger dimensions. image_imageapi_imagemagick_crop() (from imageapi_imagemagick.module) needs rework to take also the background as parameter.

After fixing #204497: Configurable background color when cropping to larger dimensions in 8.x, I will provide a patch for imageapi_imagemagick.module D7.