Rhodizonic Acid and Silver Citrate?
$\ce{Ag+}$ reacts with Sodium Rhodizonate to form a Black colored compound - but only in a neutral or weak-acidic environment. 

$\ce{Pb^{2+}}$ should give you a Blue-Violet color (or Scarlet if we're dealing with a weak-acidic environment); therefore, I don't think you're dealing with a lead contamination.

source: http://pubs.acs.org/doi/abs/10.1021/i560110a034?journalCode=iecac0