Hi,
There are these occasional black lines that appear on some of my objects. They are really annoying and I was wondering if I could get rid of them.
they look like creases in a cheaply made plastic fusion factory, and are accentuated by your subsurf.
Solution: Remove doubles. See mesh tools in wiki.
If that dosn’t work for any given bad shading line in subsurf, it’s usually messed up normals (fixed with ctrl-n).
Star Weaver is right, I take it you mirrored the object without unifying your normals.
Remove doubles first. then, you must re-calculate normals with Ctrl-N. If none of that works, go into wireframe mode[z] and see if there is a vertex in the middle of that lined area. If there is a vertex right in the middle of that lined area, as if your line was a closed circle, then delete that vertex. Be careful not to delete others!