Black designs

[Drix Productions]

I know there is a thread which kind of turned into this subject already but I thought a new header might draw more people to this one and get us some more advice.

I'm having problems, like a few people on here it seems, with black designs around the bottom of the mug. They always fade or have this weird blue line in them. I feel like I'm loosing money by not printing black designs (my friends have some really nice work they have done but they have a black border) but I have also wasted a lot of money trying to solve this problem. Someone mentioned heating up the bottom of the mug first - how do you do this? What method works best? I'm also having problems with all of my darker designs (blue/purples) towards the bottoms. Otherwise the mugs print totally fine and with really vibrant colours.
My ICC is practically screen accurate, which I don't even get with the printer I use for my stickers/prints, I'm using Sawgrass inks, Truepix paper, BMS mugs.....pretty sure its nothing to do with my consumables, just a problem with these darker colours towards the bottom. Would increasing the time/temp work or will that just burn the top bit of the mug? My settings are so perfect for lighter colours, they always come out perfect.
Any help would be greatly appreciated!