Looks like some butt checks are going to be happening
but without looking I can say my girls have kind of pink little butts - black doesn't sound right to me.... but I'll have to really look in good lighting to swear by what I just said
But...just by hearing him cry and you thinking something may be wrong sounds like a vet check may be a good idea - you know your puppy better than anyone and if you think something isn't right - I would get it looked at.