Hi
The best product I have used for the removal of black mould and the staining it produces is 'Kilrock BLAST AWAY Mould Remover'. While I don't know what the effect might be of applying it on the painted surface of your telescope I have used on a variety of surfaces including emulsion paints, oil based paints, various plastic surfaces, silicon sealant, acrylic sealants, bare plaster and have not noticed any detrimental effects to any of these surfaces. That said one of its constituents is clearly a bleaching agent similar to normal household bleach probably sodium hypochlorite. Not only does it remove all traces of the mould, including any staining, it also seems to actively inhibit regrowth especially on the more porous surfaces.
It might be worth testing some on a small inconspicuous area to see how quickly it removes the staining/spores and check if it has any detrimental effect on the coating. You can purchase as a liquid spray or brush on gel from various high street outlets or from ebay.
Usual disclaimers other than I have found it to be extremely effective and usually have some about just in case I find some mould about the place.
Hope this helps
Martin