I'd use min() to get the min in all three color channels. Then use bwareafilt() to get just the largest blob. This will be the rectangle. You can use find to find the first and last row and column and finally crop the image, something like this untested code.
grayImage = min(rgbImage, [], 3);
binaryImage = grayImage < 200;
binaryImage = bwareafilt(binaryImage, 1);
[rows, columns] = find(binaryImage);
row1 = min(rows);
row2 = max(rows);
col1 = min(columns);
col2 = max(columns);
% Crop
croppedImage = rgbImage(row1:row2, col1:col2, :);
