AnimationState.blendMode  

// Set the leanLeft animation to blend additively

animation["leanLeft"].blendMode = AnimationBlendMode.Additive;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        animation["leanLeft"].blendMode = AnimationBlendMode.Additive;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        animation['leanLeft'].blendMode = AnimationBlendMode.Additive