In Blackbird, you can isolate a specific region of the video image and apply a blur effect to that region. This is useful for blurring faces or logos for privacy or stylistic purposes. Here's how to do it:

Open Video Effect Control Interface:

Begin by selecting the video clip that contains the face you want to blur. Open the Video Effect Control Interface by right-clicking on the video track and clicking on the video effect icon.

Add Mask:

In the Video Effect Control Interface, click on the "Add Mask" button. This will create a square outline on the first and last control keyframes, and a similar shape will appear on the video, isolating the area where the blur effect will be applied.

Adjust the Mask Shape:

Left-click on the square's edges and drag to adjust the horizontal and vertical scale of the mask independently. Dragging the corner of the square will adjust both the horizontal and vertical scale together. The mask shape will also have a colour highlight for better visibility, which can be toggled on or off.

Change to Circular Rounded Object (Optional):

Right-click and drag on the mask shape to change it from a square cornered object to a circular rounded object. This is more suitable for blurring a face.

Position and Scale the Mask:

Adjust the position of the mask to cover the face by clicking inside or outside the shape. You can also adjust the scale and size of the mask as needed.

Animate the Blur Shape (Optional):

If you have a moving video and want to blur a face or object that is in motion, you can animate the blur shape over time. This allows the blur effect to follow the movement of the face or object. You can achieve this by setting keyframes for the mask position and scale at different points in the video.

Preview the Blur Effect:

Play the video to see the blur effect in real-time. The isolated region of the video should now be blurred according to the mask you created.

By following these steps, you can easily isolate a region of the video and apply a blur effect to that specific area. This technique is useful for blurring faces or objects in videos for privacy or creative purposes.