I found this about the red sap -
Many of these trading ships sailed around the Cape of Good Hope to the Orient and back again on lengthy voyages. They often stopped for water at the Canary Islands off the west coast of Africa before entering the Mediterranean Sea.
It was here where they sought out the blood of dragons. The traders bought it from the Guanches, an odd tribe of blue-eyed natives that have inhabited the Canaries for centuries. But the blood in dry crystallized form was not drawn from living dragons but from a remarkable tree that grew only in that tiny part of the world.