What version of Blynk Library did you try to install? C++ (WiringPi) or NodeJS?
C++ - Using C++ on a Raspberry Pi with Blynk
EDIT - OK, I am not a fan of the WiringPi so I had to double check if it used onoff or not… but based on what I see, and one of your comments above (that I previously missed) it looks like NodeJS.
What type of RPi are you using?
You can try to reinstall and see it that helps…
sudo npm install onoff -g