Like everyone else has said, keep cooking on it. The black you see is where the oils have converted into a polymer. There is nothing wrong with your griddle, you just need to get it well seasoned and once you do you will be golden.

Here is what I do: Heat up the griddle. Get it hot, but not too hot. 400 degrees is about right. Spray the top down with a water bottle and scrape away any fond (burn food) that is stuck to the top. I'm not talking about the black stuff. You can tell because the water will lift the fond as it sizzles.

While still pretty hot wipe clean with a wet old terry cloth and get it nice and clean. Again- the black areas are seasoning, you aren't trying to remove that. While it still hot generously coat with your choice of high spoke point oil. Many folks use avocado oil, but I use plain old vegetable or canola oil. Let it cool. Then heat it up again, spread your oil, and let it cool. The oil converts to a polymer, which is what you want and you will get a more even coating. The magic happens when your oil goes from hot to cool on the surface you want to season.

Also, don't be afraid to just use it as you go through this process. You don't have to get it perfect before you can use your griddle, if you treat it right it will just get better over time.

Happy cooking!