Broke:
Are you using the "brightener" in the zinc electrolyte solution? If so, try without it.
For me, the brightener is NOT compatible w/blackener. It gives a poor finish, and it will rub off on my fingers. On the un-brightened part, the black is fairly durable, although not 100%. See the pictures:
http://www.hogheaven.com/hobby/plati...c/cctest3.html
Have you measured actual voltage & current during plating? I'd like to know those figures if you have them.
Sean