What Eren had given Marie was a concoction derived from his blood.
Alephee had prevented Eren from using his blood directly in any potioning experiment. Especially if those experiments were going to be conducted by someone else.
She had taught him a potioning method to extract the properties of his blood that gave it its no-elemental conflict nature. The process got rid of any other attributes that made it his blood. Therefore, the concoction resembled water.
This colorless liquid would show various properties from time to time. In some cases, it would not reflect any light that fell on it, turning the liquid completely black. Sometimes it would defy gravity and not fall into the vessel even if the vial was tilted so that it was poured outside.
