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.
You can also define an event handler using the Properties and Events panel.
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.