"Blocks" are probably used so that the less tech savy members of the video game owners club can figure out how much free space they have without doing the math in their heads (which is simple to us, but hard for them). I think seeing 4 blocks on the back of a box is a lot simpler than 428 KB (or whatever 4 blocks are equal to). There is a conversion rate between the two, but I do not know what it is.