When in doubt, use blond to the masculine gender  and blonde to a girl.

   You can use both ways to refer to their hair colour.

    Her hair is blond(e)

    She is blonde.

    His hair is blond(e)

    He is blond.

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   This doesn´t mean, that some people won´t use them against dictionaries definition, mainly in spoken English.   It´s different when you are taking a test.