Printing white on black
Loris, several years ago I had the same bright idea, to prepare black paper by coloring white paper black with ink, and thought I'd share my experience here for whatever it's worth to you.
You were right to wonder whether "china ink" (by which I assume you mean something like a sumi ink, or like what is often called here "india ink") is waterproof. It didn't even occur to me to wonder; I just assumed that when the bottle said "permanent" that meant it wouldn't come off once it was dry. Imagine my surprise when I started coating gum on it, and found that the wet coating lifted the ink and spread it around, turning the white paint grey and turning the underlying black background into a smeary mess. Sizing over it would have the same effect; the wet brush would pick up the ink and smear it around, spoiling the solid black background.
I learned that there's a difference between "permanent" and "waterproof" although at this distance I couldn't explain why "permanent" is called "permanent" if it's not waterproof. At any rate, I bought about eight different kinds of ink during this exploration, and I found that if the ink was "permanent" but not waterproof, then it wouldn't stay put when moistened, as described above, and if the ink was "waterproof" then it formed an impenetrable rubbery surface that repelled gum, or anything else wet that was put on it.
Someone on the list asked why didn't I just use Stonehenge black paper (I had tried Arches Cover black and didn't like the coarse, open texture). I did order a sheet of it to try, but it's still in my flat file waiting to be gotten back to. It's not an expensive paper, less than $2 a sheet, and I know Stonehenge prints gum well, or did, some 10-15 years ago.
Just some thoughts, from someone who wasted some time and money going down that particular road,