There is a built-in fade build for bullet points in Keynote. To use it, create your bullets, select the text box they are in, and go to the Build Inspector. There, select an Effect, then go to the Delivery drop-down menu. One option is By Highlighted Bullet. This option will fade the prior bullet point as a new one comes in.
Unfortunately, you don't have a lot of control over the fade (you can make it a different colour, for example). If you need some more specific effect, you will have to create it "by hand" using separate text boxes for each bullet point, with the appropriate build-in and build-out applied to each.
There is a built-in fade build for bullet points in Keynote. To use it, create your bullets, select the text box they are in, and go to the Build Inspector. There, select an Effect, then go to the Delivery drop-down menu. One option is By Highlighted Bullet. This option will fade the prior bullet point as a new one comes in.
Unfortunately, you don't have a lot of control over the fade (you can make it a different colour, for example). If you need some more specific effect, you will have to create it "by hand" using separate text boxes for each bullet point, with the appropriate build-in and build-out applied to each.