I've done quite a few wedding quilts with guests signing blocks at the reception. Any light colored fabric that will show up a black or blue ink will do fine. I've even used a light cloud fabric. As mentioned before, no white on white fabric.
I use fine point sharpie markers. For an extra fine point, a gel point pen works great. In addition to ironing the blocks to freezer paper, I sew around the edges at least a quarter inch or 3/8" from the edge. I use an inexpensive thread and widen the stitches to a 2.8 or a 3.0 for ease of removal. This helps with keeping the guests from signing to close to the edge.
If possible, I try to tell each guest about staying inside the stitched lines, or have a sign posted on the table where the blocks will be.