""...the late-1800's are pretty easy to document." Wow! Have you tried finding documentation of particular ex-slaves in Alabama or Mississippi at that time? If you know how, please give me some tips. "
To document the lineage of a particular family to that time period would be fairly easy, however to document a particular person out of the blue would be MUCH harder. In this case, documenting the genealogy of this family might point to clues as to their John's identity. I think it unlikely that their John is THE John Henry -- family tales are rarely accurate, but it's a lead and it might offer some light on the subject. Maybe it's not really their ancestor, but someone who lived nearby and later got claimed or confused to be their ancestor.
I'd be very careful to go backwards from the existing family to look for clues. The census' from 1880 onward are MUCH more useful, and it will help develop a pattern of the neighboring familes and who might be allied to whom. The 1880 census was pretty thorough, and didn't miss as many people as some of the earlier ones did. However, I think your best bet in researching a former-slave population is to dig into church records, which are more likely to be carefully kept and recorded than the official ones. Providing, of course, that their church kept records; not all of them had reliable access to a literate person to keep those records.
I haven't done any research in Alabama, but I have done a lot of research in rural farming communities, and they present a unique set of challenges, mostly because of the lack of reliable official records. I understand Alabama is particularly poor in this department. You really have to understand the whole local history and the ways families intermingled to nail down some of the harder subjects, simply researching one family doesn't seem to work.
I'd be happy to spare the other Catters the boredom of a genealogy research how-to discussion :) I may not know Alabama, but I do know a lot of places to look for info. Feel free to email me with specific research questions, and I will see what I can dig up as possible sources for you. Eventually, you may need to refer the question to a professional genealogist who specializes in the area and in African-American ancestry, but you should cover as many bases as you can before going that route, and keep it well documented so that nothing gets repeated. It's a tough field, but not impossible.
-Nicole nicolecastle@NOSPAM.prodigy.net (Take the "NOSPAM." out, of course.)