Those blocks of flats that are repainted only several months back are getting blacken stains also...Blocks starting with 18+ .....several blocks of them. Those stains are probably coming from the corrosions of the water tank or metal railing surrounding the water tanks.
The worst are stains on top of the carpark eaves ( blks 18+...). Those stains might have generated from the roof-top gardens, either from mud/sand or manure / fertilisers.
Lets hope the authorities will look into the problem of corrosion or bad design and improve on those conditions.