Google plus is the "Google account" you need to use any of their social services.
I am frankly surprised that they allowed the login of their acquisitions to stay fragmented for such a long time.
Now they seem to force you to always have the Google+ enabled even if you never use it. Including for older accounts. I guess this is what makes people angry. It's like being prompted to create Picasa albums every time you try to read your Gmail.
Google's trying to switch over to a new system there's just one generalized Google identity (hosted by G+), and all of the products/services tap into that. If you want that identity to be your real identity, you use a Google+ Profile. If you want to remain anonymous or represent a business entity/organisation, you use a G+ Page.
The problem is there's two Google+' sharing the same name: "Google+, the identity system" and "Google+, the social network" and they've done a hamfisted job of explaining what the value of any of it is to the user.
Some social services like Youtube and Google+ require it, but you could still use Gmail without having Google+ enabled.
Unfortunately the mistake they are making is in defining every service as a 'social' service.
I am not too sure how much the blogger business is. It will not surprise me if it is folded into Google plus entirely