"Stove Black" is made by many companies (as well as home made). There are several 'basic recipes' for it, but most consist of 2 parts: a pigment, and a vehicle. The "black" is usually 'lamp black' (
carbon powder), and/or graphite. It is suspended in the vehicle which is most commonly a wax, but may be an oil instead.
Water and detergent
may cut the vehicle. You might try it to see if it cuts it. Oil, on a rag, may cut it. You might have to resort to something like Ajax/Comet for the abrasiveness to cut it. Depending on what they used, you may need all three...water/detergent, oil, and abrasives.
Good luck...it will come off, either easily, or with some work. You will need to re-season it after it is completely cleaned. All this work will probably make you appreciate it even more, once it is done.
EDITED to add:
Personally, I like to use lard for seasoning. That was the seasoning of choice 'back-in-the-day'. Vegetable oils are a new invention.