Cast Iron Newbie

Hello all. I'm new to dutch ovens and just bought a Lodge 12". It came pre seasoned, but I seasoned it one more time before cooking. I just made coq au vin and after cleaning it, I still see some black residue. The instructions says to spray it was veg. oil after use.
Is the black residue normal or should I keep on scrubbing it?
Is the black residue normal or should I keep on scrubbing it?