Sweet iron rusts & has a sweet taste. Black steel is painted with a protective coating that helps block rust & corrosion. Plus black steel bits will many times flake off some of the coating if chewed on or scratched into. 
I personally don't like black steel bits, & associate them with the cheaper quality type bits...

 


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Nightgames owes me $100. If this message is still in my siggy, she hasn't paid up yet.