I think it was...Namor? who posted a thread about it. IIRC, he used Saphir shoe polish to color a ugly gray/brown 5zip to dark brown. There's some other leather recoloring projects here on SF as well.
Found it: http://www.styleforum.net/t/285081/saphir-x-mmm-5-zip-darkening-a-leather-jacket
I'd imagine black would be easier than brown. There's specific leather dyes available as well.