Blacl/dark green streak in babies poop??

This IS NOT my picture. Husband three away his diaper before I could get a pic and took the trash out lol someone else on here had posted a year ago and this is exactly what my sons just was. So I took hers, hopefully not upsetting to her but she never responded when I asked if she found out what it Was. Hes acting fine. No solids. No formula change. He's 3.5 months. Help :( I asked the dr and they just said if he's acting fine, then not to worry. Well, I will worry anyway lol I couldn't tell if it was black or dark green. Lol