"That the difference between the beautiful and the ugly (...) has important psychological effect is a commonplace, except in the field of city planning. If it be not quoting the Devil for Scripture, Nietzsche has written: "All that is ugly weakens and afflicts man (...)"

Is "Quoting the Devil for Scripture" the reverse of the proverb "The Devil can cite Scripture for his purpose?"

Please explain the meaning (in the context) of the blue clause.

Thank you.

I think it is the same proverb in a different format, Cadzao. Presumably the writer does not like Nietzsche, but wishes to use a quote from him to illustrate or support his point.
I think the author feels Niezsche isn't quite an authority in terms of distinguishing between the beautiful and ugly, taking in account what Niezsche wrote in his works.

Thus, he will quote him, even though (even with the reservation that) Niezsche isn't an authority on such subjects, just as the devil isn't an authority on the scripture.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thank you, Mister Micawber. Your explanation makes the sentence quite clear.
 Marius Hancu's reply was promoted to an answer.
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Yes, this explanation seems much better.

That's very kind of you.

Thank you, Marius Hancu and Mister Micawber, for helping me.

But my strong recommendation, in each field, would to read the original writers/creators (say Niezsche, Freud, Le Corbusier, etc), not the commentators. Don't lose your time, life is short, go to the essence.