I've used straight opaque black, not thinned, to highlight the textured areas of clear blanks. Applied it with a brush, let it set for a bit then wiped the blank down with a damp soft rag. The hard part is applying just enough pressure to get the paint off the top surface but leave it in the textured areas. When it works right you get a very nice detailed effect. I've done some crawfish style blanks this way in order to make the shell segments more defined.
Also, you can base coat with any color...then seal the base with some light waterproof sealer, like concrete sealer or even rattle can clear, just as long as its thin. Follow that up with a dark opaque color applied and wiped off like mentioned earlier. Paint with transparent/iridescent colors in light layers. The darker details highlight the base color and the top colors create the pattern or effects.