Thanks for all advises.
I've tried Ivory Black watercolor paint in tube. It mixed very well but it's quite weak. I've added about 8ml per 100ml gelatin and tissue wasn't really black. To get deep black I'd have to use whole 15ml tube.
So I've tried to mix lamp black powder with water and it was much easier than I've expected. I've took 2g of powder and added water drop by drop until I've got nice mud. It mixed with 100ml gelatin really nice and tissue is deep black.
Unfortunately I've overexposed this tissue by at least two steps but it looks very promising with really deep black and clear highlights.
Relief is almost not visible and whole picture is completely matte. Looks like nothing I've saw already. Like very fine velvet.
It will take few more tries until I got predictable results.