Blood oozing out of Samandriel's vessel.
Angel blood is technically the blood of an angel's human vessel.
It's unknown if it contains any supernatural properties. Presumably, as with demons, the presence of an angel may change the blood somewhat as Castiel is unable to use his own blood in a ritual that requires human blood, and his blood as a fallen angel proves to be part of the Bone Of Righteous Mortal Washed In the Three Bloods of Fallen, the weapon that can kill a leviathan. It has been said that angels in the show are able to bleed out if they lose enough blood. This shows that the blood their vessel contains must have some importance to them.