Black is definitely not what you want here. Contrasting the primary CTA element with other UI elements is essential for task completion. The key tools for creating visual distinction are: position, shape & size, color, motion, and message.
This article elaborates on how to use color effectively in CTA buttons.
I'll assume that the primary CTA in this case is Join. Assuming can't alter position, shape/size, position, motion, or message, you're left with color. Don't limit yourself to having a static color. Your interface isn't static. An alternative solution to having the button colored is to keep it black until the user fills in the form fields. Then animate it to some other color to indicate that they're ready to submit.
The goal here is not to have a pretty picture, but to guide users to sign up. You'll probably be able to convince the client that having an accent color will help them achieve that goal. KISSmetrics made a great post called How do colors affect purchase? that will help you make a case. You ought to conduct a proper A/B test of the colored/black buttons if at all possible.
I also found a similar question that may help inform your decision.