hello!! why did daemon choose the surname blackfyre instead of targaryen if he thought about himself as the true heir of aegon 4?

Because he needed to emphasize the sword as making him Aegon IV’s heir. If he’s just a legitimized Targaryen, then Daeron is older. But Daemon’s political argument is that, due to Daeron’s bastardy, House Targaryen’s male line ended with Aegon IV. 

Daemon instead presents himself as the founder of a new dynasty, through being the chosen heir of the previous King and the daughter of Aegon III, the wielder of Aegon the Conqueror’s sword, and as a conqueror.