I await to be shot down in flames on this, but if you can see a "blotch", however tiny, then it is not a blind spot (because you can see it - geddit?). If you do as Air Conditioned describes, the symbol disappears, it does not turn into something else.
What he does not say is that you must have one eye closed for the experiment to work, and if you use your right eye you look at the symbol on the left for the other one to disappear.
So, if you are experiencing this with both eyes open, then I suggest a visit to an optician is called for, and as soon as possible.
Good luck.