Does opaque glass make sense?
Absolutely! There doesn't need to be anything symbolic or metaphoric about it.
There exists a manufacturing process where they mix some additional material into the glass that makes it "hazy" (usually white in color) to the point that it's barely transparent any more.
I recall seeing various kitchen equipment of this sort, like bowls, plates, cups, and in particular, heat-resistant baking utensils.
Their appearance can often be characterized with a "milky" impression, and they get decorated with moderate amounts of colorful (hand-) painted ornaments.