This warning sign does not mean that your blog is not safe or secured, nor .blogspot.com domain is considered unsafe. This is a new browser detection rule where when a web page requires to display a form with input fields, some inputs will trigger this warning if the web page is not served with an HTTPS protocol.
The input field that will trigger this new web browser ruling are:-
input type="email"input type="password"
If both of these input type is available on a web page, and the web page is not served with a HTTPS protocol, the "Not Secured" warning sign will display at browser URL bar.
How to Fix This Warning?
On Blogger blog or any web page, please enable HTTPS for all your web pages/URLs. Of course for blogspot.com blogs, the HTTPS feature is absolutely free & no SSL certificate needs to be pruchase or installed. Users only requires to enable HTTPS from the blogger dashboard > Settings > Enable HTTPS section.
For websites, you need to buy an SSL certificate & verify the authenticity your hosting provider. Best to contact your hosting provider to find the best SSL certificate to purchase & use for your website.
For Blogger blog custom domains, users does not have the HTTPS options enabled (yet), so below are the quick fixes.
Fixing "Not Secured" Warning on HTTP Web Pages
If your blog or web page displays this warning, you can replace the above input type & use an
input type="text" instead.Depending on the location of your form input fields, find these input attributes & replace them. On Blogger blogs, it could be in your
Theme > Edit HTML or in a dedicated Pages > click Edit > select Compose Mode > find the input fields that need updating.Sometimes we find Blogger template developers requires to insert form fields using Blogger widgets. You can have a look inside these widgets to see if any of the "flagged" input fields & update them accordingly.