I've had this before with my Bishop!
My juice would go really dark and dirty looking too. Initially i increased the air flow and it went away. But more recently I swapped from Kanthal to Ni80 and its never happened again and I reduced the air flow back down to where I was happier.
I deduced that Kanthal has a slower ramp up and a longer cool down and therefore carried on vaporising after I'd let go of the fire button. And then that bit of vapour went through the wicking hole into the tank.
Ni80 and SS coils seems to give more of an on/off heat and its cleaned up my Bishop.
Thanks @Deadanddripping. I think I'm allergic to nichrome so have to stick with the kanthal. I might go back to building my own simple 1.0 ohm 28 GAs as the claptons seem to get clogged with gunk very quickly with the liquid I use and have a noticeably longer ramp-up.