When there’s too much light being let in when trying to capture a motion blur, a few recommendations are as follows:
Lowering the aperture – this then automatically decreases the amount of light that is let in the camera in the first place.
Decreasing ISO – this would decrease the camera’s sensitivity of the image sensor
A Natural density filter – the article referenced explains it would be as if you put sunglasses on the camera, allowing it to adjust to the light and still use slow shutter speeds.