Are you talking about the line drawn diagonally and then sewn 1/4 inch to each side??
If so, you're going to cut right ON that line so it'll virtually disappear....doesn't really matter what you use to mark it. I've used ballpoint pens, pencils, Sharpies, water-washable pencils, even an awl to 'just leave a line'. Never a problem.
I request sharp ballpoint pens for the classes I teach using HSTs; they can also use them for taking notes!
Also, you may find that if you hold the block down with a firm press on a ruler, then start your marked line in the middle of the block out to one corner, then from the middle out to the other corner, you'll be less likely to distort or move the fabric.
Jan in VA
Last edited by Jan in VA; 01-03-2015 at 09:23 PM.