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The described behavior doesn't occur when blending two 32 bit Forms.
Would you elaborate on your scenario? How do those lower-depth forms come to be these days?
We use Pen, which is a subclass of BitBlt. More specifically, we use Pen>>roundNib: among others to create a circular brush for painting on a Form. This method creates a Form with depth 1. One can then set the color of the brush with Pen>>color:, which calls BitBlt>>fillColor: which in turn sets its halftoneForm. According to the class comment of BitBlt, the pixel values of the halftoneForm are ANDed with the sourceForm pixels before the blending rule is applied.
A workaround for us might be to use Pens with 32 bit sourceForms, if that is possible. I only noticed the different behavior of 32 bit forms while trying to create a code snippet to reproduce the problem, so I haven't tried this workaround yet. Also, I think that the problem still occurs when using alpha values between 1 and 254, even if those might not be as noticeable.
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