yea thank you very much for all these great responses...
I will fix the bug as soon as possible. I used the app always with my camcorder so i didn't realize the bug. Thank you!

I fixed the bug with small resolutions...

download new version at:

www.ultitech.de/en/bluebox.html

Do you work in RGB colorspace to determine blue-color class pixels? Try using HSV or better CIE-L*a*b instead, they are better suited to distinguish colors regarding their crominance and luminance.

yes i worked with RGB.

But thanks for the suggestion of working with HSV. I will look for that!!!