Thanks a lot for sharing this shortcut, it works! If you are using your own Blynk server, add 8080 after the IP address.
Update: By the way, you need to create two short cuts, in my case I create one to open my garage door, and one to close it. If security is a concern, you can name the short cuts to anything, like" Open my den, Open my cave, I want nachos, etc.
This is really handy, I just tell Siri a block away from my house: Hey Siri open garage, she replies: That’s done. Love. it!