Well, I see something like "blending sketch" firtst time, it would be helpful if you could link up these tutorials But right now all I can think of is blending the sketch with bg or just blending colors on your sketch(if you start up with colors as underpaint)...Dunno, i can only guess.
I don't remember what tutorials, unfortunately... But I'll try to look in my bookmarks, and see if I added any of them I think it might be what the artist is doing in this speedpaint (though I'm very unsure, and it goes by pretty fast): [link] It begins about 1.50.
Oh, I see ^^ Itried something similar before, but it didn't turn out very well. I think I now have figured out how to do it better though: I changed the colour of the sketch to one more similar to my colour scheme and then I blended it the way you said. Turned out way better, and much less messier than when I used black