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Contrast vs opacity using gabor

I built a QUEST staricase using a gabor patch to estimate an individual’s threshold for 75% 2AFC detection which varies the contrast to do so. However, it does not work with a black background. However, with a white background it works. For using a black background, I found that adjusting the opacity makes this work. Contrast just doesn’t blend the gabor in with the background enough to work. It’s still visible even when the contrast is -1 and the background is -1. My question is, why is it that only opacity works? I need to be able to explain in my experiment why I am using opacity and not contrast. Even better, to find a way where I can use contrast for my experiment rather than opacity. Any links or explanation would much be appreciated.

I don’t really understand this stuff, but is this thread relevant?

Thank you very much! They keyword I was missing is luminance.