This has been discussed many times and should be in the archives.

However, blood in the milk is common enough, I think, and I don't
think anyone knows why it occurs, really.  It should not stop the
mother from breastfeeding, and will usually clear up within a few days
to a week of starting breastfeeding.

Why is the mother so concerned about the baby getting blood. It is
true babies tend to spit up more when they drink blood, but milk is
"white blood".  Besides the baby is getting blood from her right now,
in utero.

Jack Newman, MD, FRCPC