I'm not sure what a darkimakura is, but we have "daki-makura" here in Japan, which most people use as a pillow to cuddle up with. "Daki" means hold and "makura" means pillow in Japanese. They come in all shapes and sizes and most are just plain pillows shaped like small punching bags.. There are some however that have anime pictures or depict anime characters that I have seen.
I'm not surprised that such a personalized version of a dakimakura exists for men. Many Japanese men here would rather have a cyber girlfriend, blow-up doll or watch cyber porn than actually go out and find a real flesh and blood woman to have a relationship with (including my brother-in-law). It baffles me why so many men are inept at interacting with women here. But if that pillow or blow-up doll fills that need for them then who am I to judge?