Color.black  

// Assign a black color to this transform's material
transform.renderer.material.color = Color.black;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
transform.renderer.material.color = Color.black;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
transform.renderer.material.color = Color.black