Hi Adilcan. I'm also a great fan of 'black basing' and 'marbling'. I hope you don't mind me replying to Craig, But this is a great way of getting tonal variation into models - especially those that are predominantly one colour usually grey). The best site I've seen for it is by the (in)famous Doog, link below...
Technique: Black Basing
Four days to get to this point. I really do wish I could model like that! Great work - I really do like your style.