For signing blocks I like to use Pigma pens, 08, or 05 nothing smaller. I went to a baby shower and the grandmother to be says to me "Oh I meant to call you about a signature quilt". I have an awesome hubby, and I had just washed and ironed some bleached muslin so I was able to call him and tell him exactly where it was, with mat, ruler and cutter he brought them to the shower.
Remember to mark the boundries where they can sign, some will still sign in the seam allowance:)