Do you think maybe it's the disclosure about self promotion you added? You explicitly say the purpose of the blog post is to promote your consultancy, so that might be why they marked it blogspam. I know it feels like you're being forthright, but really that you're promoting yourself is implicit in the fact it's a personal blog, so you can leave that out and still be honest.

Yeah, maybe it's that, though I still wouldn't expect someone to categorize the post as blogspam, even if they just glance at it. (At least according to my definition of blogspam, but I guess each has their own.) But yes, pragmatically I should probably remove the disclaimer.