Blood red wine

In ancient Greece undeluted red wine was reserved exclusively for gods and was only used for libations. Drinking undiluted red wine was considered sacrilegious and in some cities was punishable by death...

Greeks also believed that blood was the favourite drink of the dead. The dead liked blood because blood is life and only through drinking blood could the dead obtain living consciousness. When Odysseus goes to the underworld to talk to the dead he first gives them sheep blood...

The red wine was the closest thing to blood, which is why eventually blood letting (blood libation) was replaced with the red wine libation...

In Serbia the ancestral cult, the cult of the dead, is at the core of all folk beliefs. Dead were believed to have power over the living and could decide the faith of their descendants. Basically all the dead were deified... Even the main pagan god of the Serbs, Dabog, Sun (Sky) god, was considered to be the progenitor of the Serbs, their original ancestor. He was also the god of the dead, the god of the underworld....So no wonder that in Serbia, during funerals, and during annual visits to the graves, undiluted red wine is poured on the graves of the dead. 

As a matter of fact, in the past, no feast could start without the dead being mentioned and some (originally) red wine, but now any spirit, been spilled on the ground "for the dead"...

As I said in my post "Thirst" Balkan Slavs believed that droughts were caused by "thirsty" ancestors, who were not given enough water through libations by their descendants. 

Destructive storms were also seen as the "wrath of the angry ancestors". In some parts of Serbia people believed that storms actually contained the demons of the restless dead. 

To protect the livelihood people would cut their fingers and point the bloody side to the clouds...

Why? Were they giving their own blood as the sacrifice to the dead to appease them?

Ancient Greeks in the same situation were lifting read scarves towards the cloud. This is again a replacement for the much older blood sacrifice for the dead which was unchanged preserved by Serbian peasants...

That Serbian peasants preserved this most archaic sacrificial ritual is not surprising at all. Blood sacrifices were a big thing in Serbian culture. Agricultural year started and ended with a blood sacrifice. A new house couldn't be built without a blood sacrifice. The end of the harvest was marked by a blood sacrifice...

More info about South Slavic blood sacrifices can be found in my post "Kurban"...