Stand-alone flashing of STM32
You're looking for a bootloader. Luckily the Bluenrg series comes with bootloader developed by ST. You can refer to AN4872 for the details.

From the document


  The BlueNRG-1 and BlueNRG-2 bootloader is activated by hardware forcing high DIO7 at device reset.


Take care that the Bluenrg is a 3.3V MCU (If I remember correctly), you may need to additional hardware changes.