Printed pages have colored spots

Re: Brother HL4070CDW laser duplex printer. I am having problems with random sized and placed blue spots on printed pages. They are arranged in no particular pattern. I have taken out the cartridges and turned over the "drum" to clean with a q-tip, as instructed in the manual. I notice there are indeed flecks of blue toner and I remove them. I print a few pages and the spots appear again. I cleaned again and spots come back. I called Brother and they instructed to press "back" and "cancel" at the same time to clean the drum. This seemed to work for maybe a hundred pages and now the problem started again. I have tried all of the above methods of cleaning again, but the problem continues. This all started after I installed a new blue cartridge. I am using the high capacity cartridges and they are EXPENSIVE, so I hate to take it out and put in a new one. But maybe it is the cartridge that is defective and not the machine. What do I need to do?