Apple has started an iSight camera replacement program for the iPhone 6 Plus, offering to replace defective camera modules in certain iPhone 6 Plus models that were sold between September 2014 and January 2015. From Apple:

Apple has determined that, in a small percentage of iPhone 6 Plus devices, the iSight camera has a component that may fail causing your photos to look blurry. The affected units fall into a limited serial number range and were sold primarily between September 2014 and January 2015.

If you own an iPhone 6 Plus and have started noticing blurry photos from the camera, you can go to the support page to enter your serial number and see if you are eligible for a replacement. If you are eligible, you can have the camera module replaced at an Apple Authorized service provider, Apple retail store, or by contacting Apple's technical support.

Source: Apple