2 Blacks on 240V?
 

I'm putting in an electric water heater and the water heater has a red and black wire, along with a ground. However my house only has 2 black wires and a ground, I traced it back to the box hoping to maybe find a red stripe somewhere, but they are black all the way to the breakers. On 240V does it matter which way they're hooked up as long as the lives are hooked to the lives and the tank properly grounded?

It does not matter which is which.

Thankyou! That puts my mind at ease.