Keep track of array class and data scale when working with images. You likely have integer-class data, but you're storing it in an output array that was implicitly preallocated as 'double'. As a consequence, the data is handled by imshow() and imwrite() as if it's a unit-scale (i.e. [0 1]) float image, when in reality, the data is scaled to the original integer class (e.g. [0 255]).
The easy thing to do is just specify the appropriate class when allocating the output array.
crop = zeros(row_spacing,col_spacing,3,blocks,blocks,class(rgbImage));
