Freepatentsonline.com has been helping people search patents for over 20 years, which puts it in a different league than most new sites. Its technical security is modern, and the fact that it shows up on blacklists nowhere is a good sign for a site this old. But there is a catch you should know about before signing up.
Most legitimate services that require you to create an account—like Google Patents or the USPTO's own tools—tell you who runs them. Freepatentsonline.com does not. There is no about page, no team information, and the domain ownership is hidden behind privacy protection. That opaqueness is unusual for a service that collects user data.
The site also has an SSL certificate that needs renewal in two weeks. While that is a minor technical issue, it adds to the overall feeling that the operator is not paying close attention.
If you are just browsing public patent data, you are probably fine. But if you are considering creating an account or uploading any sensitive materials, ask yourself why they do not want to say who they are. That lack of transparency is the main reason to treat this site with caution rather than full trust.