Is it really black or just a very dark blue? Ferricyanide reacts with iron ion to form two different compounds that are very dark blue. In any case discard the solution as its bleaching ability has been compromised. Pour it directly into the drain and follow it with plenty of water. Rinse the container well too. Any other disposal method is less safe. Ferricyanide solutions are one of those that should be protected from light as this hastens its decomposition. Many people prefer to mix up fresh solution just before use and then discard it.