I have had the same although I would have said very dark brown rather than black spots, at around the same age and still get occasionally - I am sure I read somewhere was fine, as I have not done anything about it - will see if can recall where I read it. I never thought they looked particularly like blood though, but did wonder why most is mustard yellow why certain bits dark.

I did find this on a website - but this isn't where I read it before -

Q: My baby's stools have black spots in them. Why is this so? His stools are dark yellowish in colour. He is also constipated. I gave him banana powder mixed with formula milk. Now what should I give him?
Rinkie (Forth Worth, USA,)

A: The stools of your child are normal. You must wait till your baby is 6 months before you give any outside food.