Well there do exist dvd recordables with gold bottom. OLD MITSUI/MAM-E (made in japan) and MAM-A (made in america/japan.)
Mitsui/MAM quite the production because the dye could not handle high speeds (2x was probally the fastest!).
I think I should have some pictures of this media somewhere.
These days MPO does use gold on DVD media however they combine it with a purple coloured dye. Which gives a rare shade of purple.
DVD's are more complex sure but it shouldnt be too much of a trouble to make black or silver if you as a manufacturer really wanted to, no?
It's much more complex as you think. To find a dye which is colourless and works at high speed and at the correct wave length.
As far is a I know only MITSUI found a dye which was colourless however it wouldn't work with high speeds like I said befor.