Blend Glitch C issue?

I was running several mix windows as keystone windows in order for them to be transparent as they overlap in places.

This all went well.

I then started playing with blend types and found blend C to work really well with this setup. Unfortunately this led to any area where 2 mixes overlapped ceasing to be transparent.The upper mix became black background wherever it overlapped another mix with blend C. Change the blend back to A or B on either mix fixed the problem.

This was using build V4.4 m364

A fix I used for this is to apply an FX designer colour effect to the mix, and have the FX designer do transparent black - that turns anything black on the mix to transparent, so when you have no layers visible, the mix is invisible.