They never confirmed if Black was losing power or not (SSGSS's stamina drain). What makes the Completed SSB different is the SSGSS's burst power, which is a different characteristic of the form.
The explosive power it's like when you turn on the lights and it uses more energy for the first seconds before stabilize to a lower level.
The stamina drain is like a light that can't be mantained too much because its energy decreasses every second.
And the manga never said shit about Black's explosive power nor stamina drain as SSGSS.