Looks like some dried blood to me--possibly internal. Do you have other roosters? If so, it probably came from fight or peck or maybe just from hitting something. Just keep an eye on it and look for an avian vet if it gets worse. and doesn't clear up .
Yes, we ended up with 3 roosters even though two of them were supposed to be pullets. I didn't intend for any of my chickens to end up as dinner, but the boy in the picture is somewhat of a bully. We are building a new coop that will give everyone a ton more space, so hopefully things will get better. Thanks for everyone's help.