Hi,check and make sure urself,that dot is from lens(both front and rear) ,otherwise u need to clean ur mirror.how to clean the mirror? remove the lens, and for me i use very few optical cleaning liquid drops to cotton bud,wait a while and then swap it softly. But recommend only there is visible dust on mirror surface. i never try to blow,bcoz that dot may goes inside to CCD may cause u more problem.
Another option,if ur dslr is very new and under warranty,go to service center.