Is your black more like a brown and looking a bit blurry? If so then we've found that when printing with Ricoh SubliJetR inks you can't over cook the mug. From what your reporting you would think that you need to press for longer to get the black, but it's the other way round. The brown blurry blacks are caused by too much heat for too long. Take the press time back to 3.5 minutes at the recommend temp.