"Why not just report them to Google?" I've done exactly that. Unfortunately, the report process for "impersonating a business" actually requires you to be a representative of said business. I had to report them for some other reason and just mention in the comment box that Blogger itself was being impersonated. Being that Google is a giant faceless corporation that doesn't pay any attention to anyone if they don't have bags of money to throw around, I don't expect anything to come of it.
The domain registrar, while being the long shot I've already admitted that it is, is still a much more likely avenue of success. Below is the message I sent them via their abuse report page:
This company is spamming my blog on Google's Blogger service, and likely many other blogs as well. Thankfully, I have comment moderation enabled and none of their spam comments are actually visible (because I've been manually marking all of them as spam). They are impersonating Blogger in the process of doing this, and Google gives me no avenue to report them for it. I realize this is a long shot, but I simply seek to strike back against them in any way I can.We'll see what happens. If you're actually reading this and want a follow-up, I'll probably just edit this post. There may not be a follow-up, though, it depends entirely upon the domain registrar's response, if I even get one.
Their profile on Blogger (the link "My Web Page" links to <domain redacted>, I used an internet whois service to find your site): <link redacted>
Even though the comments were never published, I get email notifications of comments on my blog that contain the full text of each comment, and can forward any number of them to you if necessary. This has been going on since June 2016, basically since they registered their account on Blogger.