What is a water blank?
Generally a blank means no sample and just solution. So it would not contain the 10g of soil in this case. I think for soil analysis it's used as a control so you can have something to compare to.

Here's some links I found by doing an internet search.
http://asl.irri.org/lims/index.jsp?page=about&amp;feature=soilanalysis

http://www.soiltest.vt.edu/PDF/lab-procedures.pdf