when does that black prenancy line appears on your stomach?

First, it's not black, it's just darker than your normal skin color. Second, not everyone gets it. With my first I had a very, very faint line. I didn't get it with my second or third pregnancies. It's just a hormonal thing and it usually appears in the first trimester, but it can appear in the second as well. Generally speaking, if you make it through the first two trimesters without it, you won't get it. I forgot that was something that even happened until I saw a commerical for Sister Wives and saw Robin's belly.