Adds an event handler that runs when the element loses focus.
An element loses focus (blurs) through user actions,
such as clicking and tabbing, or programmatically, using the
blur() function.
Note: onBlur() has no effect when applied to RadioButtonGroup elements.
handler(event: Event): void The name of the function or
the function expression to run when the element loses focus.
This method doesn’t return any custom errors, but may return standard errors. Learn more about standard Wix errors.