Blynk have helpfully provided a detailed explanation of why the expiry of an SSL certificate was responsible for the outage, and why only a subset of users were affected:
The certificate was updated automatically 2 days ago. Old Blynk firmware had a bug where the intermediate certificate footprint was hardcoded. The new certificate has the intermediate certificate changed. That caused connection issues for the hardware.
Why not all users where affected?
Some of the devices established the connection before the certificate was updated, so they were able to connect. The connection is keep-alive and everything was ok. Also, during the certificate update the server is not restarted, so connections with prev certificate were working fine. Only new connections weren't working. That's why we weren't able to detect this issue by our monitoring system.
The fix is the next - right now we serve 2 certificates. One is for hardware, another is for web and apps.
That should solve the issue. Sorry for inconveniences.
Blynk have definitely exceeded my expectations in their response to this issue. I am very impressed as we pushed the boat out developing the R24 with this industry leading technology and they have proven themselves to be a very good business partner.