* Torrential Tribute activates during either player's Main Phase.
* Synchro Summoning occurs during the Main Phase.
* Torrential Tribute targets face-up monsters on the field.
* Black Rose Dragon is not on the field when it's Synchro Summoned. It is summoned directly from your hand or field to the field.
Therefore, your opponent cannot target Black Rose Dragon with Torrential Tribute during its Synchro Summon. They could activate Torrential Tribute during a later Main Phase, if Black Rose Dragon is still face-up on the field.