I am looking to patina a material I believe to be zinc based, Zamak 5. Will any Jax products either darken it in a similar manner to cold blue on steel, or, alternately, make it iridescent and colorful?

The Jax product you will find most useful is the Jax Pewter Black. You should be able to achieve a dark grey to black patina on Zamak 5. A few experiments with application methods and dilution will be helpful.