In a 2005 article in Atike Kunst, Gunnel Ekroth examines vase paintings to explore the role blood played in Greek sacrificial rites. One main aim is to support his thesis that Greek reserved sacrificial blood in order to consume it, and he defense this view by examining when blood is and when it is not shown on vases.
Ekroth concludes the essay (26-7): “In scenes of thysia sacrifices, i.e. sacrifices ending with a meal for the worshippers, we see no blood at all in representations of the divine part of the ritual, even though the actual killing is occasionally rendered. Blood pouring from the victim was apparently not considered a central part of the iconography of the thysia.”
One factor is the presence of the sphageion, a vessel for collecting blood, which, he says, “can be found in depictions of the pompe , occasionally in connection with the altar and when the victim is killed, but there is no representation of blood being collected in the sphageion or of its being used to pour blood onto or at the altar. Instead, the sphageion occurs most frequently and prominently in scenes showing the mageiroi preparing the meat, a context referring to the human part of the ritual, which centred on a meal. The iconographical connection between sphageion, mageiros and food preparation is best taken as an indication of the blood’s also being kept, cooked and eaten.”
In short, “worshippers benefited from the blood more than the gods did,” a point supported “by the fact that altars are never shown covered with blood, but only with a few bloodstains on the vertical side, probably representing the sprinkling or splashing of a small quantity of blood only. In addition, the stains are not found on all altars and there is not even a preference for show- ing altars with bloodstains in scenes of sacrifice. Even though blood was sprinkled on the altar at every animal sacrifice, it was not necessary to depict it, and the vase-painter may instead have chosen to leave the front of the altar blank or to add a garland or some kind of architectural element.”
There may be other factors guiding the depiction of blood “but the occurrence and appearance of the bloodstains certainly supports the suggestion that most of the blood was not poured over the altar at actual animal sacrifices.”
Flowing blood is rarely depicted, and only in the context of sacrifices other than the regular thysia. Flowing blood seems incompatible with sacrificial meals: “The spilling of blood seems in fact to dissociate the ritual from any form of meal. Sacrifices which focused on the blood of the victim and at which the meat was usually destroyed, such as oath-taking or purification, were seldom chosen as motifs on vases. It is interesting to note that when such rituals are occasionally found, as in the case of the battlefield sphagia , at which the bleeding seems to have been the essential element, the flow of blood has been toned down to a small trickle. Freely flowing blood is confined to representations of sacrifices which are not only set in a mythical context, but also have little or no correspondence to actual, practised rituals, such as the revitalization of the dead or the sacrifice of human beings.”
In a footnote (11, n. 11), Ekroth notes that the prominence of blood is one of the main differences between Greek and Hebrew sacrifice: “Although many similarities can be found in the sacrificial practices of Greek and Hebrew religion, the differences, particularly those pertaining to blood, have not been sufficiently taken into account. In Hebrew religion, there is a strong blood taboo: blood represents life, belongs to the divinity and is therefore not to be consumed by man. There is also a prominent link between blood and atonement, which is only found in some Greek sacrifices with a particular purpose. Furthermore, the central importance of blood in Hebrew sacrifices is evident from the fact that priests could only execute some blood rites, a practice not paralleled in the Greek sphere.”
(Gunnel Ekroth, “Blood on the Altars? On the treatment of blood at Greek sacrifices and the iconographical evidence,” Antike Kunst 48  9-29.)