They've come up in the news in the past week....mostly because AOC says she will target them (in the 2020 primary deal) and help to get true-Democrats into their seats. So, some history deserves to be reviewed.
The term 'Blue Dog' Democrats really didn't get uttered until you get to the mid-90s. You may think it was the Reagan-era, but you'd be incorrect.
Kin to the 'Boll Weevils' (southern Democrats of the mid-1940s to the late 1960s)? No. The Blue Dog's come from across the entire US.
You might want to note that they aren't relative to the Dixicrats or the Bourbon Democrats (1800s).
They tend to be from districts which are mostly middle-class, and fairly divided on values/problems. Some folks would suggest they are 50-percent Republican and 50-percent Democrat. I would suggest otherwise....that they found issues that aren't really owned by one party or the other, and got voted into office because of some argument they presented on two or three important subjects.
AOC's desire to wipe them out? There are a total of 27 currently in the House, and there might be five to ten more throughout the US who are either leaning that way or avoiding the label while they act that way.
I suspect that three problems would come out of this AOC threat.
First, there is the total lack of understanding on the 'balanced' nature of voters in the US, and how some folks are marginally voting Democrat and could be persuaded with some effort to flip their votes to some Republican.
Second, out there and waiting for opportunities...are the Green Party guys. You might upset the system enough that five to ten of these seats could go to a 3rd party mechanism.
Finally, if there was a major House loss in 2020....blame might go around in such a major degree....that the Democratic Party might split into two groups. AOC would probably prefer not to create that big of a mess.