Yep, the Col-Erase is right. I draw pretty heavy-handed, so its easy to have a dark line for me. Its a light blue shade too, which is different/darker than a non-photo blue. Maybe you should go a little darker, if its hard to see.
I use different pencils all the time, but one of my favorite graphite pencils is this one, The Palomino Blackwing pencil. Based off the Blackwing pencil of the 40s that the Disney animators used to use. Its a soft graphite, but nice and dark line.
Tom, I really enjoy seeing your process and blue roughs. I'm always worried that my sketch work isn't readable enough because I don't have a steady pencil stroke, but seeing that you find your lines before committing is really awesome and helpful. Can't wait to get your new book!