getPhaseGradientColorMapBlackColor() {
    const p = this._gradientMapTransitionPhase;
    const ps = p * p * (3.0 - 2.0 * p);
    const ips = 1.0 - ps;
    const fb = this._fromGradientColorMapBlackColor;
    const tb = this._toGradientColorMapBlackColor;
    return {
      r: fb.r * ips + tb.r * ps,
      g: fb.g * ips + tb.g * ps,
      b: fb.b * ips + tb.b * ps,
      a: fb.a * ips + tb.a * ps,
    };
  }
Gets the current Gradient Color Map black color point accounting for phase.
@return     {cc.Color}     color   The color value.