You can get different thicknesses.
Another brand that is used are Sharpies ... can be purchased in office supply stores. Also available in different widths.
Will those at the wedding understand what the signature blocks are all about?
Years ago, we were at a wedding, and they came around with the blocks for us to sign. No one at our table understood what it was about or what to do. Long before I had heard about signature quilts!
So as a suggestion .... maybe consider a written sheet explaining what it's about, maybe even with a picture?
Wonder if there is a way you could attach the block to that sheet and use it as the stiffener for them to write on?