Due to the elastic nature of the BlazeMeter API Monitoring infrastructure (more details here), we do not publish lists of IP addresses for whitelists. Between the regions that we use and depending on the load, you could see a wide variety of source IPs at any given time.
You can get a known source IP address for requests in your tests by using the On-Premises Agent on a host you control. The agent also allows you to send test requests from within your own infrastructure. Once running the agent acts just like any of the cloud locations and can be enabled within a test or shared environment.