Managing Your Work Environment > Setting Preferences > Timeline Preferences >
Blend

Set the default for working with blends.

Default Blend Mode box
  You can set the default Blend mode when you create blending transitions.

Select: | To:
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Fade In Blend | Create a mix curve that starts at 100% at the first frame and ends at 0% at the end of the clip.
Constant Blend | Create a constant mix value of 50% for the length of the blend.
Fade In/Out Blend | Fade in to the incoming clip for the first 0.5 seconds and fade out for the last 0.5 seconds. Clips shorter than 1 second fade in for the first 25% of the clip, and fade out for the last 25% of the clip.
Fade Out Blend | Create a mix curve that starts at 0% at the first frame and ends at 100% at the end of the clip.