Need help identifying a children's book, 10 points for helping, please?

The book is a children's book about two girls who live in an apartment building in New York or its suburbs.

I read the book in the 1970's or early 1980's but, the book in fact seemed to take place maybe in the 50's or 60's.

The older daughter had a snooty friend named Cynthia who had borrowed a blotter but came to return it as a peace gesture (I recall this because I didn't know what that was). The mother in the book had gone to the store and had purchased "peacock blue" curtains (again, I remember because I didn't know what color that was). The mother had said something to another shopper like, "You might have seen them first. lady, but I'm buying them first." and then the mother had bought them and the girls loved the color.

I googled this already and couldn't find it!