Sign in to the Zoom mobile app. While in a Zoom meeting, tap the More option in the controls below the screen. Select Background and Filters (iOS) or Virtual Background (Android). Pick Blur from the menu. Your surroundings will become concealed by the blurred background.
If you do not have the Blur option, make sure your mobile app is up to date, you have the required hardware, and you have checked the device's processor generation.