tls_config:

  insecure_skip_verify: true

Sounds like you're using a self-signed certificate ("signed by unknown authority"), so the Blackbox Exporter doesn't know to trust it by default.

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You can either disable TLS certificate checks (which comes with a security penalty) by setting the "insecure_skip_verify" option to "true" (see https://github.com/prometheus/blackbox_exporter/blob/3a08e67780bcb728a42cbf00ba03e0bd363a8e8e/CONFIGURATION.md#tls_config) or you can install your private CA on your Blackbox Exporter host so that the Blackbox Exporter trusts certs signed by it.

​