That’s correct. The USB serial connection to the Blynk server needs to be used exclusively for Blynk communications - you can’t use it for anything else.
This is why many code examples use SoftwareSerial to create an additional port that can be used for debugging. However, for your PC to be able to use this port it will need a TTL to USB serial converter, often known as an FTDI adapter.
In reality though, the USB serial connection method is only really useable as a way to get people up and running to ‘play’ with Blynk if all they have is an Uno with no other method of connecting to the internet. Your long term solution should be to abandon this connection method and go for a proper IoT device…
Pete.