I found a tip on You tube where the guy suggests rolling a length of toilet roll into a sausage and laying it along the sealent,you then soak it with bleach and leave it for 24 hours.
The toilet roll keeps the bleach from drying out.
It removed all the black mould from my shower sealant.Using bleach and wiping or brushing it didn't work,this has worked a treat and has saved me the task of re-sealing.