According to what I have read, there are problems with ingesting human blood. Questions like this indicate that mar'is ayin applies.
During metzitzah in a circumcision, the mohel takes some blood into his mouth but then spits it out. However, according to some scholarly sources, there was a practice in the middle ages of mixing that spitted out blood with wine and having the parents and/or the baby drink some.
Here is one quote (and I don't know his sources, but I will also quote a corroborating historian)
After the mohel performs metzitzah, he traditionally spits the child's blood into a goblet of wine or sloshes the blood and wine in his mouth and expectorates into a cup. Then the mohel dips his finger and gives the child a taste. The boy's parents next take a sip, followed by the mohel. . . Today, most Jews sip only wine. But they once drank blood. . . Sipping the circumcision wine as wine rather than blood also dilutes the symbolic potency of the rite. The Torah , to repeat, firmly prohibits the consumption of blood, one of the premier laws incumbent on all of humanity and not just Jews. Yet paramount ritual remembrance of the covenant, with all its promise of progeny, requires participants to violate this decree by drinking a child's "life force." Eric Kline Silverman, From Abraham to America: A History of Jewish Circumcision, p. 138.
In Germany in the high middle ages additional customs arose concerning the blood: the circumciser would spit the blood of metzitzah from his mouth into the cup of wine from which the baby would be given a few drops to drink Shaye J. D. Cohen, A Brief History of Jewish Circumcision Blood (from, The Covenant of Circumcision: New perspectives on an Ancient Jewish Rite, edited by Elizabeth Wyner Mark).
And there are a few others. Now, I haven't checked these texts to see if their scholarship is right. They might all be extrapolating from a single anecdote in a single medieval text. But if the claim is even remotely true, wouldn't it have presented a halachic concern? Is there any source for this practice or discussion of its halachic status? Are there any sources which debunk these claims?