I have a shadow gyarados and it need a shadow physical move. Shadow chill is very unuseful and hurt by reverse mode every turn. So i think if add a shadow move tms seller that the shadow poke can learn more shadow move. Maybe can let all shadow poke much strong. English isnt my first language so maybe the grammar have some error.
I get what you are saying but not every shadow Pokemon has access to the same shadow moves. So Shadow TM's would be like normal TM's, if the Pokemon can't learn that move then the TM wouldn't work. Great idea though !