I would, personally, keep the blood magic and use it as an excellent conversation starter between party members. Someone could comment that blood magic shouldn't work on something that has no blood. Another could hypothesize that oil is kind of like blood, hence why the magic works. And yet a third could say that maybe it's not reacting to blood, literally, but the life force/sentience that is within the robot.
You could take it a step further and have it only work on the tank robot (and not machinery in general), suggesting that there's something extra special about him. That makes for great characterization.
Lastly, it can make for one awesome quest. Imagine the party briefly heading out to figure out a way for blood magic to work on the robot. Maybe they need to find a special vial that can be integrated into the circuitry. And then they need to find a rare monster with blood potent enough to trigger the magic, yet can still fit into that small vial.
So, I wouldn't abandon the idea. Just find a logical way to make it work.