Hello and welcome to Petco Pet Education Center, formerly Petcoach. The picture isn't the greatest quality so I hope I am seeing what you are seeing. The black at the ends of the nipples are normal and not of a concern, as is a little bit of a different colour near the belly button region. If there is a bump that is sore, swollen, red or hot where the belly button is and it is discoloured, this is not normal and very concerning, but this doesn't appear the be the case in the picture. Best of luck with your little one!