take off the mag, then spray from the lens port side through the aperture of the gate. on cameras like the arri SR the dust will be blown out of the camera, on other cameras like 535 or BL the dust will be blown into an area which is much more accessible than the mirror shutter area.
you will be able to get the particles out of there in an easier way.
if you try to blow out the dust from the other side it´s probable that some of it remains in the area between filmgate and mirror shutter, therefore there will be a risk that it sticks to the gate again and you´ll have hairs etc in your frame. not very nice.
dirt on the mirror shutter negatively affects the quality of the image you see in the viewfinder. it´s not only the black spots you see in the image, a dirty mirror leads to loss of brightness too. so, do as kris and david told you!
cheers
t.