Hi
It is true that the mirror and the focusing screen are probably the most delicate parts of the mirror chamber and can be easily ruined. It is also true dust can be removed from there without even the slightest chance of damage.
Please remember any dust you find there was not attached to any of those surfaces with some sort of an applicator. This dust, at one stage, was actually floating around inside the chamber, floating because it is very light and small in volume. So if it floats like a feather it settles on the surface like a feather, do not forget this fact. The only thing it holds it there is surface tension and static and that can vary depending on the makeup of the dust. So if it is as light as a "feather" it can be removed with a "feather" if you understand what I mean.
There is one proviso; If you place the camera pointing to the sky with the lens mount open crap will find its way inside the camera which could be more than just fluff.
The reason you cannot remove the offending dust with the rubber ball blower is this:
The strength of air coming off the end of the blower is actually very weak. You will need a concentrated jet of air pointing directly at the dust. This is next to impossible because the nozzle end of the blower dances wildly in all directions as you squeeze the ball. Also to mess around with blown air in these parts of the camera is not a very smart thing to do. You will blow more dust into than out off this part of the mirror chamber. In fact I do not like blowers at all, the never remove dust but relocate it - always! I have my blower deposited in the little round filing cabinet under the desk
So how do you safely remove the crap. Simple. Buy from an art supply shop a very very soft artists brush. (Nylon perhaps as this will hold a small static charge) They have them, some are so soft you couldn't tickle a flea with it. But make sure the brush is
not "sized".
Don't forget that dust can't be seen if it is on the mirror surface so you don't really need to clean there.
Now gently without bearing down brush over the focusing screen from left to right (or from right to left) in one pass and the offending matter will be gone. It will be stuck on the brush. If you feel the need to go over a second time don't be tempted to flick the brush with your fingers, it will pick up oil or sweat from you skin and this will be transferred to the surface of the screen and then you will be in trouble. Instead flick the brush with sharp movements over a pencil or such. (Did I say clean pencil?)
There is no way there will be any damage left behind. It is simply not possible to do damage with a very soft brush.
If the dirt is of an oily nature like Salami or Cheese, (Just kidding) or sticky pollen then the above method will not work, You will have to remove the screen and wash it, but this will be another story. I can walk you trough this also if you are interested.
Greetings