How do I know if a website has been blacklisted?
It is the most severe sanction ever imposed on a website. Google doesn't impose it for a minor issue.
A website is blacklisted if it no longer has any indexed page and therefore no longer receives any visit from the search engine.
Test your domain name using the "site:www.votresite.fr" and "info:www.votresite.fr" commands.
If in both cases, you don't get any result when you previously did, then it is very possible that your domain has been blacklisted.
If you use Google Search Console, you may have also received a message warning you of the sanction. By the way, it is through this service that you will be allowed to ask for a new evaluation as soon as you will fix the problems responsible for the blacklisting.