Image processing: Detecting "black patches" in a "white area" (with Java)

Question

My problem is to detect the black "holes" in BMP images like the following:

https://www.hmmueller.de/Diagrams_x560/image0001.bmp

Here is another example:

https://www.hmmueller.de/Diagrams_x560/image0002.bmp

To find the black "blotches", I first simple create a "greyscale" image by adding R+G+B. In the hope of having a bimodal distribution for this values, I used a Java algorithm for Otsu's method (https://en.wikipedia.org/wiki/Otsu%27s_method), but the resulting threshold of 59 is apparently way off - here is the actual distribution, cut off somewhere below the maximum R+G+B value of 255+255+255, which has an even higher frequency count:

https://www.hmmueller.de/Diagrams_x560/DistBMP.JPG

Practically, I'd put the threshold at 500 - but is there a better way to find a threshold for the "obviously dark blotches"?

(From there on, I'd use a flood fill that finds all contiguous areas below threshold; and returns bounding rectangles for the blotches, which is good enough for my further needs).

Thank you! H.M.