Is it possible to connect a leonardo via USB and use this to control the PC as a hid mouse and keyboard over the app?
Yes, I think this is possible.
I’m not sure if Leonardo can act as HID and Serial simultaneously, but I hope it can…
You will have to check the USB Serial example: https://github.com/blynkkk/blynk-library/blob/master/examples/BoardsAndShields/Arduino_Serial_USB/Arduino_Serial_USB.ino
You will also need to write virtual pin handlers and use Mouse and Keyboard objects of Leonardo.
If it goes well, could you please post this sketch here, we would like to add it to our examples?