Corn meal and corn starch are 2 different things. Corn starch should have no black dots. Corn meal (coarsely ground corn) could have occaisional black or brown dots -- pieces of plant matter or a tiny windblown seed that got into the pile of corn seeds. Maybe only 3 or 4 black dots in a whole cup full. Corn flour should not have any visible black dots -- they would be ground too fine to see.
No the iron would not appear as any kind of dot. It would be invisible.