Been enjoying cooking with cast iron and have gotten better at cleaning and seasoning. My usual routine after cooking is to put a few tablespoons of coarse kosher salt with a little bit of water and use the abrasive sludge to scrub off any food or oil buildup with my fingers. I then rinse it off and try with a soft towel and then reapply a thin layer of avacado oil.

However, the last couple of weeks the pot is retaining a dark black buildup - if you cook pancakes, they come out grey instead of golden. Eggs have a blackish/gray tint to them.

I scrubbed with a pumice stone and then put it through another 4 rounds of seasoning in the oven, but it's still pretty black.

What should I do to remove the black buildup on it?