the black card is not transparent, no matter how fast you move, it still record the card in a very faint image,
to cut down more light reach the sensor, you need to record more of the black card
in the end you get a very low contrast image.
So, just .......... get ............. a .............. ND ................. filter.
Anyway, black card doesn't exactly substitute GND. It blocks light from going to the sensor, while GND reduces light going on the sensor. Movement effects will be more pronounced with GND/ND filter.
For example, if your exposure is 30s and your sky is 1 stop over for that exposure, you need to bring the sky down by 1 stop. If you use a 1 stop GND, your exposure in the sky will be 30s. If you use black card to block the light, your sky will be a 15s exposure while your foreground will still be 30s. If you don't mind/want movement effects in the sky then the black card will be good, but if you want it, then you should get a GND filter.
of course a GND will be convenient but black card can sometimes offer more flexibility.
Usually, I would spot-meter both the sky (highlights) and the foreground of a scene before using the black card to compensate for the difference in the difference in exposure. Of course, using an ND filter would help greatly in this aspect. For example, if the meter for the sky reads 1/200 at f/11, and 1/15 at the same aperture for the foreground, I'll pop in a ND-filter (usually a Hoya 9-stop) to give me an equivalent exposure of 2s for the sky and 27s for the foreground. I'll usually over-exposure the foreground a little bit more to get the exposure a little more even. It's actually a pretty straightforward technique and you should try it out! Cheers.
Thank you so much for all the great inputs