It suggests that the company no longer wants Google+ to represent it around the web. Google's brand is much stronger than that of its social network, and some developers may be more comfortable adding a generic Google login to their sites and apps. Given those facts, the blue button may be better for Google as it competes with Facebook, Twitter, and others for single sign-on supremacy. But rolling it out will only reinforce the perception that the best days of Google+ are behind it.
As I've already said, I have a bad feeling about all this. I really hope that Google won't kill off Google+. Yes I know some representatives has said that they will keep going forward with their plans for Google+, but I guess we have to wait for Google I/O to actually see the truth on all this.