Dark Sunflower center?
There are several possible reasons that one sunflower may appear darker than the others. The flower may be more mature (older) and the seeds more developed. It might be a natural genetic variation in the cultivar.; or it could be a different cultivar that got mixed in the packet.
If there are no other signs of pests or diseases, there is nothing to be concerned about. These little variations are part of the joy of gardening.