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. 
Doesn’t it depend if you’re using the word as a noun (in which case it’s opacity) or a な adjective (in which case it’s opaque)? It can be either.
Edit: to add to that, the “measure of how much light can pass through” meaning is not the only definition of opacity. It can also just mean “the quality of being opaque.” And that’s the meaning of 不透明 as a noun.
If you email them they’ll probably add opaque as a synonym, but I don’t think it should replace opacity.
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