It's hilarious that you think Google actually signs up to blog networks to find out their pages.
Google is not a person. Google is an ALGORITHM.
They don't have armies of people signing up for SEO services so they can find out more about them. They're a mega company. They don't concern themselves with individual blog networks.
They have algorithms to identify greedy networks and patterns. The fact a few people are finding their rankings drop from only having blog networks could be for any number of reasons. You can't conclude Google has some team signing up to BMR then writing articles and buying links. That just shows a complete lack of understanding about how big companies work and think. They couldn't care less about some shitty little blog network. They're focused on the big picture.
It's more likely the following:
1) They've tweaked the algorithm so link diversity is even more important. Perhaps specifically taking in scenarios where links are almost all blogs.
2) They've improved the greedy network identification algorithm. To be honest it's not that difficult. I could update their algorithm to identify "blog network" standard type links. The blog network sites are generally very obvious and full of patterns.
It's hard to look at all blogs on the Internet and out of that lot identify blog networks, but it's not so hard to look at someone's backlinks and answer the question:
Is it likely most of these are from blog networks and/or really low quality blogs?
The reason for this is that if they have 1000 backlinks, and 900 of those are blogs, they can look for patterns within those 900 blogs. Contrast that with trying to find patterns between all blogs on the Internet. This, among other reasons is why link diversity is so important. Not just wider link diversity, but link diversity among your blog networks, ie don't just use one blog network, use multiple.
There's a number things that make that last question very easy to answer and there's a number of things you can do to make it impossible for them to answer, but I'm not going to talk about them publicly, you'll have to figure that out for yourselves.
Suffice to say anyone who says "blog networks are dead" automatically gets put into the "doesn't have a fucking clue book" in my eyes.