A customer bought the Olympus OM-D E-M5 Mark II yesterday, and noticed that there is a blue pixel in dark photos. So he visited us again today, and I confirmed that the blue pixel is there.
Googling the problem online, and I found that many have such issue with Olympus cameras.
Solution? Run the pixel mapping in the menu of the camera. Problem solved.
Official Olympus Pixel Mapping explained: