Is it known whether smurf blood is red or blue?
I checked the UNICEF ad (an ad where the Smurf village was annihilated by warplanes), but couldn't spot any blood in the ad.
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Officially, the color of smurfs blood is not known, but there is enough information to conclude that they have red blood.
As a comic targeted to young children, any display of blood is carefully avoided. As an example, any harm is displayed as dark marks:
The real reason why their skin is blue is unknown. Some have supposed it's because their blood is blue, but it might be caused by pigmentation.
I thought I remembered one of them blushing because of Smurfette, or maybe it was her? Anyway, I did not find any proof.
The only thing that might indicate they have red blood is that their tongues are red, which is caused by hemoglobin on mammalian. However, Smurfs are not mammalian, but it's still a good indication of their blood color.
I found a blushing Smurf here (french site), it's the #90, with the legend "90 Schtroumpf rougissant," which translates to "blushing Smurf". The image was taken from number 1354 of Spirou magazine (published in 1964). It is canon as it has Peyo's signature :
As you see, the blushing Smurf is purple. So red blood + blue skin = purple Smurf.
As kylusD indicated on Reddit, there is an episode (Smurfette's Rose) were Smurfette wishes to Mother Nature for a Blue Rose, but it made her turn white. However, she pricks her finger to break the spell:
Her blood was red!