The news has been full of stories in recent weeks of how Anthem Blue Cross has been trying to increase health insurance premiums by as much as 39 percent for some people who buy insurance on their own, outside of the kind of group coverage a person gets from an employer. Almost entirely unnoticed, meanwhile, has been a more newsworthy development: in a time when medical costs have been rising rapidly, premiums for many public sector workers enrolled in a Blue Shield of California HMO have actually gone down.