I keep hearing people saying that, but checking again this seems to be the only relevant section:
So yes, blackened Servants can hold back, they just don't normally bother -- and if they do, they lose fine control.Also, as they no longer bother to restrain their magic energy consumption, their fighting ability jumps significantly. This momentum can be compared to a runaway train.
Because they are in that state, fine control is not possible, but in exchange, their destructive power increases.