Can you share two examples of an image with a black border? It seems that you have to check the color of the pixels of the image. Here is an example of how you can access them:
using (MagickImage image = new MagickImage())
{
PixelCollection pixels = image.GetReadOnlyPixels();
Pixel pixel = pixels[0, 0];
// With pixel.GetChannel you can access the channels of the pixel, e.g. RGB:
int red = pixel.GetChannel(0);
int green = pixel.GetChannel(1);
int blue = pixel.GetChannel(2);
}