Question

Where can I find the Tm aerial ace in black?

Answer 1

Mistralton City Runway after talking to a Youngster in the house north of the Pokémon Center after defeating Skyla.

Source: Bulbapedia/ Mistralton City

Answer 2

Talk to this little boy in the house next to the airport in Mistralton City. After you show him that you earned the gym badge, and he will tell you that he has an item on the Mistralton City's Runway. Go to the Runway and look around you will find the TM.

Answer 3

After you defeat Skyla at the Mistralton City Gym, go speak with the little boy inside the house directly above the Pokecenter. Do this only after you beat Skyla. Speaking to the boy will allow the TM to appear in the the bottom-left corner of the Airport runway in Mistralton City.

Source: https://guidestrats.com/pokemon-bw-tm40-aerial-ace/