I was just going through my The Ultimate Book of Everyday Knots and found this knot, the Figure of Eight Noose. It works fantastic for Blocks. If the Blocks should somehow fall out, which happens a lot since they’re so small, all you have to do is to loosen the noose and put the Block back in.
Here are 2 Blocks that I just tied using this knot. Probably the best knot to use for this purpose.