Mineral pans are high carbon steel pans, like a wok. The metal will discolor with heat. I agree with scrubbing the pan with Bar Keeper's Friend. Make a thin paste of barkeeper's, and water. Spread it evenly on both the inside, and outside of he pan, and let it sit for 15 minutes. Then use a sponge cleaner to scrub under hot water. When the pan is again shiny, rub a thin layer of oil to coat the inside of the pan. lace upside down on a parchment, or foil lined cookie sheet, and into the 400' oven for 1 hour. Turn off the oven, crack the door, and let cool with the oven.
To season on the stove top, place dry pan over medium-high heat. Use a paper towel to evenly apply oil to the inside of the pan. It will start to pool up toward the center. Rub with paper towel to even it out. Apply about 3 coatings of oil to the pan, allowing each to harden between coatings.
Finally, to get the pan as seasoned as it can get, deep fry something in it, with the oil reaching 370' F.
For cleaning, with the pan just warm, remove any grease with paper towels, and scrub with a sponge under hot water with salt.
That dark circle is probably caused by the oil pooling in the pan center, and polymerizing. Your whole pan will develop a dark patina with use. The inside of my cast iron, and high carbon steel wok are very dark, almost black, and are virtually non-stick.
Instead of using my oven, I seasoned my cast iron over charcoal on my Webber Kettle. It kept me from smoking up the house.
Seeeeya; Chief Longwind of the North.