Controlling a Stepper and it’s position, whether by counting steps in an open loop or using an encoder in a closed loop is all down to the code and various Stepper libraries.
None of that is Blynk related. Blynk only comes into play after the fact as the user interface.
Google about for all the various codes and examples on controlling steppers with Arduino…
Be aware that steppers require precise timing to control them, and that introducing Blynk can and will required use of Blynk Timer and timed functions instead of the “arduino way” of dumping everything in the loop… it can get tricky, but doable.
