Question about cast-iron pots, black specks, and roux.

Had a bunch of leftover crawfish here in Dallas and decided to make an etouffee. Started to make my own roux which I've done a few dozen times and--on the first batch--noticed the dreaded black specks which means you burned your roux.

Was puzzled because I had on low heat and stirred evenly but did not want to take chances.

Washed out the cast-iron and started again.

Few minutes in noticed the same thing.

I then remembered I had a similar problem when doing some specks on it a few weeks ago (i.e. getting black specks on the fish).

I think something is wrong with the cast-iron and need to season it again. But in the meantime want to know if there is a way to tell if this second batch of roux is ruined. Does burnt roux have a burnt smell?