The Validator itself does not affect the behaviour of RadMaskedTextBox and its HideOnBlur property.
It is possible that you have set SetFocusOnError="true" for your Validator, in this case when you try to blur, the validator gets back the focus to the input element of RadMaskedTextBox, and since the input is in focus, it will again display the mask.
Make sure you have set SetFocusOnError="false" for your Validator to avoid this problem.
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