How to change color of all non transparent pixels to black?

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I use OpenCV (Java) and I want to change color of all the non transparent pixels in my Mat to black (colors defined in ARGB_8888 but could be another format)

How can I do that ?

    Mat mat = new Mat();
    bmp32 = bitmap.copy(Bitmap.Config.ARGB_8888, true);
    Utils.bitmapToMat(bmp32, mat);
    // and then ????

Thanks !