You could use a Blynk timeout timer, or start a regular timer then delete it, but that might be a bit too difficult with your current coding skills.
The other option is to use a library like “Ticker”. If you google it you’ll find information about how to install the library and how to start and stop a ticker that will flash your LED and sound your buzzer.
Take a look at this bit of code where I use Ticker to flash an LED when required: