Your description was not very accurate. The image is not black, it simply is the wrong colour, with the background showing black instead of white.
That is because, this is an indexed image. The pixel values are indices into a colour map that you're not loading. The white colour in the colour map is at index 0, so if you use 0 as intensity instead of an index in the colour map it is going to look black.
To fix this you need to load the colour map and pass it to imshow:
[ToCrop, map] = imread('iteration 31.png');
Note: use imfinfo to check the colour type of the image.