The fiddler for the fabulous Cajun band Balfa Tojours explained this very well at a workshop at the Sioux River Folk Festival a few years back. I've of course forgotten the particulars over the years, but the basic spirit of it is this: there isn't really a yes/no answer to your question. Zydeco tends to be more the music of African-American French in Louisiana, and "Cajun" music tends to be more the music of White French in Louisiana. But there's a great deal of overlap of performers, styles, and aficianados. There are plenty of Black French who love and play the more "Cajun" style, and vice versa.

I also kind of get the impression that "Zydeco" is kind of a twentieth-century stylistic/marketing label used to describe a kind of folk/ethnic music with roots much older than its name, not unlike "bluegrass" or "reggae" or "salsa."

Anyone with more extensive knowledge of this than me, though could probably answer your question much more accurately than me. Fortunately, the Mudcat is just the place for same, I suspect...

Chris