Custom white balance works by comparing the red, green, and blue channel levels in a neutral reference and then correcting them to match. A black object is a poor reference because all three channels are near zero, so tiny noise or small color tints cause huge errors in the channel ratios. That can easily produce a strong color cast, such as green.

Also, many “black” objects are not truly neutral black; they may have a slight warm or magenta tint, and white balance correction will push the image toward the opposite color.

For custom white balance, use a neutral target that is brighter than black but not so bright that any channel clips. A neutral gray card is ideal, and a clean white target can also work if it is not overexposed. The key is that the target must be neutral and exposed within range.