Yes, it’s called spalted wood. That means that one or several fungus has/have infected the wood. Without going into details, the growth of that fungus can be stopped by drying the wood before the action the fungus has become fatal to the wood. Your wood looks quite good, but, in general, that kind of (infected) wood is only used for purposes where the wood is not put under stress conditions (bowls and furniture for example).
People that have used that wood successfully for making bows (me included) are lucky that the fungus’s action had not been fatal to the wood. But it’s a game of “give and take”. If you’re lucky, the bow wont break
Good luck to you.