My question is: how can one report this anonymously? I was planning on sending an email with links and screenshots of everything in this blog, perhaps even calling an HR representative and telling him/her what the blog is.
how can one report this anonymously?
Send the relevant information on a document to HR via regular postal mail, do not send the information via email). Do not include a return address, and pay any mailing fees with cash. If you have to supply a return address, use the HQ address in the Netherlands for that.
This should get the job done and protect you from being the one who reported. If you want to be extra cautious wear gloves when dealing with the paper.
It is the only way I can think of to report it without being potentially named as the accuser, especially if the place you work at has a bunch of security cameras.