During the creation and editing of the Media item, if the URL you are using is HTTP rather than HTTPS, you should see a blue warning message like the following:
If you tap on the Learn More link in the warning message or on one of the tiles that failed to load the media item, you would be taken to the following page with more details describing how to resolve common issues with “Mixed Media”:
Note that if you are not using a valid HTTPS URL with Blue Iris, then each browser would need to have the Mixed Content settings adjusted accordingly.