Usage invokes insecure SSL

Not related to SSL, it’s more likely that a plugin on your install doesn’t allow SVG files to be used. I would start investigating there.

Done so, installed default wordpress out of the box.
When plugin Fourteen Colours is activated the error occurs:
Insecure URL: https://<domain>/blog/wp-content/plugins/fourteen-colors/pattern-dark-inverse.svg

plugins_url() will get the URL based on your WordPress settings, so the other possibility would be that you’re missing the s in https:// in you site URL settings.