I keep a packet of blue corn flour on the shelf in the kitchen just for fun! And it is real!
It is ground from blue to black mealies, shown here. The fun thing is that the mealie-pap made from blue corn meal is purple. Commercial corn chips are also made from this flour and they are also dark purple.
Taste? No difference from white or yellow mealies as far as the flour is concerned – I have not yet seen or tasted a fresh blue mealie to eat off the cob.