Don't know if it will help, but worth a try....get a bottle of cola (any cheap make will do), pour it into the bowl and leave for a couple of hours, or as long as possible.
Hopefully when you flush all the nasty balck will go too.
It removes the limescale build up, hope this works for you!
i would try a specific lime scale remover too. leave it in for as long as you can before flushing (over night) and then clean the loo in your usual way
this may sound odd - but saw it on 'how clean is your house' and it really works! but a few denture cleaning tablets into the bowl and add a kettle full of hot water. leave for a couple of hours.
Hi - Ive worked on How Clean is your House? and in fact you'll see me on it again Tuesday 28th November 2006 when they show the re-run of the fulfords special. When they show brief glimpses of cleaners, im the only male with dark hair, then im shown again near the end dusting off the old books....
anyway what you need is a substance called ACID WASH for something that is black and leave it on for about 1hr. The black is just a build up of scum from where it is not cleaned at all in ages.