In this Tutorial we will remove the green screen and black background in transitions (e.g. Stock Footages) like in the video below, using After Effects.
Get this transition here
Let’s start by adding our green-black background transition on the timeline (called by us Transition).
Below it we add the video that will replace the green background (called by us 1 Video).

In the search for the effects we look for Keylight and apply it on the Transition layer.

Let’s apply the Keylight brush to the green screen of our composition.

We get the first result, that is the replacement of the green screen with our video (1 Video).

Now let’s move on to replacing the black part of the transition.
We duplicate the transition level (Transition) and do the Pre-compose, clicking on the right mouse button on the level.
Check the second box in the window that pops up and click OK.

Let’s add the second video on our timeline (called by us 02 Video), the one that will replace the black background.
We put it under the previously created layer (called by us “Transition Comp 1“).
In the Track Matte section of the 02 Video layer, let’s choose the Transition Comp1 layer.

The result obtained will be this

Let’s drag the Transition layer above the 02 Video layer.

Finally we choose the blending mode of the Transition layer by clicking on Screen

In this way we concluded our transition.
If you have problems, you can watch this Tutorial in the video below.
You can download the transition in this tutorial here Transition Tutorial
Find many more transitions here Kmeel Stock Transitions