Dry-Tab Phosphate tests


I bought a Dry-Tab Phosphate test kit the other day, and have tested my
aquarium water several times.  Each time the solution remained clear (as in
zero phosphate), but there is a dark blue precipitate at the bottom of the
tube?  Does anyone know if this is normal, and I indeed have very low
phosphate, or something is wrong with the tests?  Two things that may be
affecting the results are: 1) the kit is about at the expiration date (I
didn't notice when I got it), and 2) my water is pretty hard, GH=18, KH=8,
pH=8.1 (lowered to 7.4 with CO2).

Thanks for your thoughts,