This threw me when I was learning it, and going back and checking the difference between the meaning of “opaque” and “opacity”, it seems that “opaque” would be the better translation of 不透明.
To wit:
opacity = a measure of how much light cannot pass through
opaque = not transparent (ie 100% opacity)
Yeah, I know that I can add this as a synonym for my own benefit, but opaque and opacity have different meanings, so changing this would help avoid other people being confused as well.