There is a much simpler way.
Put a jumper across the Tx snd Rx pins on your board and echo every piece of serial data to Blynk like this…
The jumper needs to be removed before doing a sketch upload via the serial cable, but can stay in place if you are using OTA updates.
As it says in the link, I use a variable to control whether the serial output should be echoed to the terminal, which is set via a button attached to a virtual pin, but for testing purposes you could set the variable to true when it’s declared.
Pete.