Honestly, if someone who has known my child and our family for any length of time decides that someting as superficial as the child having blue hair in the summer is a reason to keep the kids apart I would probably figure "good riddance" as there must be a mulittude of much more serious issues we would totally disagree aboutwhen it comes to kids anyway. I AM concerned with the parents of my chidlren's friends thinking they are well behaved, polite, etc. but not what they think of their hair style.
In a perfect world, I would agree with you, but what about the family that doesn't know her very well? What about the new girl that she would like to make friends with?
I don't buy that line of thinking. I still wouldn't care what other parents think. Most children adopt their values and views from their parents, so if they want to form opinions and judge someone based upon a nine year old having blue hair, then in my opinion I wouldn't care what they thought because I wouldn't want to associate with them anyway.
While the opinions of others may not matter to your or I as adults, children often feel quite differently. So much depends upon the culture of the town the child lives in. If she is going to be way out of the mainstream because of a hair color choice, that is probably not a decision she should make. If she is going to blend in with her peer group, it may be more acceptable.