It's slightly better to specifically connect the other end the cable connected to the black side of the dead battery last, and to connect it to the frame of the car with the live battery instead of to the black terminal in that car.

The goal here is to make the last connection, the one that completes the circuit and can generate sparks, away from either battery, because lead-acid batteries can sometimes release hydrogen gas, which can cause fires or explode. The chances of this actually happening are pretty low, but there's no reason not to be careful. The end of the black cable connected to the running car is the only one that can be attached away from batteries, so that's the one used.