The peak current of 6A for the driver implies that the xPRO v5 can run stepper motors with up to a 6A rating (theoretically).
But since you need to extract the full output (torque at high speeds) of the stepper motor, ideally the peak current of the driver should be 1.4 times that of the stepper motor.
Using this thumb rule, you can use a 4.2A stepper motor (NEMA 23 and some NEMA 34 stepper motors) with the xPRO v5.
And you can use up to a 2.8A stepper motor with the BlackBox controller such as this highly popular 269 oz.in NEMA 23 stepper motor.
However, even with the xPRO v5, the power that you can supply is limited to 24 volts.
High torque NEMA 23 motors such as this 425 oz.in NEMA 23 motor requires 36V of power to deliver the high torque you need at high RPMs.
Powering it at 24V is fine but it will limit the torque you get.
For knowing how much torque you can extract out of a stepper motor at each voltage and RPM, you need to refer to the torque vs RPM chart of the motor.
To know if you'll get higher torque out of your stepper motor with the xPRO v5 compared to BlackBox, you need to see how much torque the high torque stepper motor can generate at 24V, which is the maximum recommended power for the xPRO v5.