Do you mean the transition from metalic blue scales to black scales??
If so then yes that is normal. But he is crashed on the bottom in the picture... is he usually at the bottom of the tank??
I just read in your other post that you recently brought him home, I wouldn't be too worried about him, he looks healthy. Did you add water conditioner to his tank? Some water conditioners will help fish's slime coat which is good when you add new fish.
Good luck
My bettas head is slightly darker than the rest of his body too. I dont think there's anything to worry about if he's swimming about OK and is eating well etc.