You can use a grayscale image as
a gradient wipe. In a gradient wipe, image B fills the black area
of the grayscale image and then shows through each level of gray as
the transition progresses until the white area becomes transparent.
Gradient wipe source image (far left) and resulting transition
In the Effects panel, expand the Video Transitions
bin and the Wipe bin inside it.
Drag the Gradient Wipe transition from the Wipe bin to
an edit point between clips in the Timeline panel.
Click Select Image, and then double‑click the file you
want to use as the gradient wipe. The image appears in the Gradient
Wipe Settings dialog box.
Adjust the softness of the transition’s edges by dragging
the Softness slider. As you drag the slider to the right, image
A increasingly shows through image B. Click OK.
Note: To change the gradient image or the softness, click
Custom in the Effect Controls panel.
To preview the transition, drag the current‑time indicator
through the transition in the Timeline panel.