[Python] OpenCV: understanding the filterByArea parameter used in SimpleBlobDetector

I am trying to detect a large stain using OpenCV's SimpleBlobDetector following this SO answer.

Here is the input image:



I first tried working with params.filterByArea = False, which allowed to detect the large black stain:

However, smaller spots also ended up being detected. I therefore toggled params.filterByArea = True hoping to enforce a criterion on object area.

However, when setting params.filterByArea = True with params.minArea = 10 the largest stain is no longer identified:

I tried using other minArea parameters to no avail, even trying a minArea of 0, which should be equivalent to no filtering at all.

What am I missing here?

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