Blynk change properties of GPIO

Question

I have a small problem with the python library of BLYNK: I need to set the properties of an appbutton with

blynk.set_property("G20", "offBackColor", "#000000")

It only works with

blynk.set_property("20", "offBackColor""#000000") 

with a virtual pin. I followed some tutorials from github, pypi and their own tutorials but nothing worked.

I use Python3 and a raspberry pi 4