If you're not sure that you can isolate your Black Aztec corn from your neighbors' corn pollen by distance (a few miles) or by tassel & pollen shed timing, whether theirs is GMO or hybrid or even a different open-pollinated heirloom variety, then you could consider hand pollinating the portion of corn you want to save by bagging it. Here is a link to a website with good pictures of the process:
A good book about saving seed of corn and other heirloom vegetables is Seed to Seed by Suzanne Ashworth.
Renee Pereault 
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