Is TLS 1.3 supported in Blazemeter?
Blazemeter SaaS
TLS 1.3 requires at least Java 11, so java version of your test should be set to Java 11 and the JMeter version to 5.3:
Also user.properties should be modified to read https.default.protocol=TLSv1.3
And that file should be uploaded to the test scenario as well.
However, there is a bug as described in https://issues.apache.org/jira/browse/HTTPCLIENT-1967
It is taken care of in Java 12 which is not yet supported in Blazemeter.