You'll build this project's blue and black background by using a solid and animating the Ramp effect. Follow these steps:
At time 0:00, create a new comp-size Solid layer named Background using any color. The effect you'll apply in the next step will override the solid's color.
Apply Effect > Render > Ramp to the solid.
Set the effect's Start Color to a vibrant shade of blue (RGB: 100, 0, 255) and the End Color to black (RGB: 0, 0, 0).
To start animating the effect's color gradient, add a keyframe to both the Start of Ramp and End of Ramp properties at time 0:00.
Set the effect's Start of Ramp to 160, -50 and its End of Ramp to 160, 0. Now the gradient's starting and ending colors begin offscreen, and the comp is filled with black.
Go to time 2:00, and change the Start of Ramp to 160, 120 and the End of Ramp to 160, 240.
At time 0:10, the effect's start and end colors are just coming onscreen.
At time 1:10, you can clearly see the icons (circled plus signs) that mark the effect's Start of Ramp and End of Ramp positions.
The final position of the solid's Ramp effect