I have also been noticing similar issues with involving delayMicroseconds() to control a servo (also on an UNO R3) while using Blynk… Where the same sketch works perfectly outside of Blynk. I came up with a band-aid fix using Blynk.syncVirtual() but that only worked if the delayMicroseconds() call was in a Blynk Loop… and it brought other issues, so not really recommended 
As my Ethernet shield died awhile ago, I have been using Blynk’s USB link, and thought that might have been part of the issue, but now I think not so much 
Basically I suspect delayMicroseconds() might be getting interrupted at times due internal Blynk timing.
Also, while you don’t appear to be using the analog pins, a known issue with some Ethernet shield clones was that the REF pin was shorted to either ground or 5V and thus caused unreliable analog reads. I had solved that issue by just bending the REF pin out of the way.