It is a bit confusing! ;-)
The idea is that a limited number of us (I used 9 purely as an example) each make an additional number of blocks out of blue fabrics, and send them to obq, getting the same number of her blue blocks in return. So if we make two each, send two, and get two of hers back (the ones coming back from obq would the same).
We would still get the original 18 red/green/cream blocks that we were expecting.
My thinking behind limiting the number of participants for this one was so that we got more than one blue block (so you can actually make something out of them) but so that no one had to complete another 18, which I don't know if I could.
Oh, and the block could be either what we've already done, or a new one, that's no big deal really. :-)