Hi All, just seen the thread and comments. The reason for the description on the website is that as the mug is black then any colour lighter than black won't be seen on the mug. Its like trying to print a colour onto a black tshirt - you won't (or shouldn't be able to see it).
The guidance is fairly generic to all suppliers of black panel mugs even if it isn't explicitly stated on their sites. These have been sold in the UK and Europe for many years now and the advice hasn't changed as to print area and blead around image.
The mugs however have to be fully coated as sublimation spray coating can't be applied to just a White area of a mug so in practice the whole mug is coated as a normal mug would be in a normal spray process.