I had to search many hours to solve the problem with the installation of onoff on my RPi 3 B running on Jessie (armv71).
Running blynk-client works but I get the error : No direct pin operations available…
The reason it failed is that there are errors during the install of onoff. The module epoll returns warnings and errors. Something about permissions to make directories.
What for me finally worked was running the command:
root@raspberrypi:/ opt# npm install --unsafe-perm -g onoff