You most likely got seed that was treated with a fungicide. Most commercially vended seed is treated. Many mail order companies that cater to home gardeners sell untreated seed although those who also vend to farmers will offer a choice of treated or untreated seed. The treated seed are better for emergence as it prevents rots and other fungal attacks on the seed itself. Once the plant is on its own, it does help but is not a cure all. I suspect that you had an attack of smut as the corn was earing out.

Here is a link that might be useful: corn smut

hilary, I treat the corn that I save for seed with malathion powder to keep the weevils from eating it up.

Shot