You can paint it. There is special high heat paints made for fire boxes. Also once you have a few fires it will be black inside and you probably won’t notice it.

will it really blacken with gas? i thought it would only blacken with a wood burning fire

overall, it looks unfinished as it's now. The firebox color doesn't look bad, I dislike how the legs look more. Is that dark charcoal sample what you want to use? I would look for another option in a multicolor slate that brings some of the color of the brick.