Use Blur to apply and customize a blur filter to a clip.
To access the Blur menu, use:
Timeline, then use ConnectFX (See Creating ConnectFX).
Tools, then select from the menu.
The Modular Keyer, then select a node from the Node bin.
This node accepts a front and a matte clip, and outputs a result and outmatte. The outmatte clip can have a different level of blur than the result clip.