Auto selecting square blob area from image : Computer Vision

I am trying to develop auto calibration plugin, for which I need average pixel value value of centre blob in the image. As of now I am converting this image to binary, and able to identify different blobs in image.

But, I want that central blob getting identified some how. Maybe we can take help of surrounding 6 small blobs.

Original Image: https://drive.google.com/open?id=0B9kvfpiOcM1EWnhJeGtBZ3A4eTg

Matlab Code:

I = imread('BLOB.TIF');
Ibw = ~im2bw(I, 0.75);
Ifill = imfill(Ibw,'holes');
Iarea = bwareaopen(Ifill, 500);

stat = regionprops(Ifinal,'boundingbox');
imshow(I); hold on;
for cnt = 1 : numel(stat)
    bb = stat(cnt).BoundingBox;
    rectangle('position',bb,'edgecolor','r','linewidth',2);
end

Answer 1

[Image Analyst]

Use

Iarea = bwareafilt(Iarea, 1)

To extract the largest blob.