Leaving empty cast iron pot on a gas stove I left empty enameled cast iron pot on medium high flame on gas stove top for about half an hour to an hour. It was actually filled with water which should be brought to a boil but i forgot about it, so after water evaporated it stood empty on a stove top. I don't see any cracks or any other signs of physical damage, it only darkened and it smelled burnt. I washed it with regular dish soap and use abrasive sponge (which I usually don't) but and enamel is still dark grey inside (it was white before).

I've read about other users similar experiences and it turned out not to be a big deal, but those pots weren't enameled inside. Since mine is I wonder is it health safe to continue using it for cooking. Any chances of something toxic could leak into food or something similar that could be health hazard.
I think you should be safe, but depending on the abrasive pad (plastic or metal?), you might have scratched the enamel.

Try the different methods to clean a burned enamel cast iron; most methods used boiling water with baking soda.

https://www.thekitchn.com/how-to-clean-burnt-stains-off-enameled-cookware-cleaning-lessons-from-the-kitchn-217457

or 

https://www.epicurious.com/expert-advice/how-to-clean-stained-enameled-cast-iron-pot-article