While I think you're correct that it may take some time for the newly sighted to be able to process the visual signal at all, the question as stated mentions:

  "... the blind man, at first sight, would not be able with
   certainty to say which was the globe, which the cube, 
   whilst he only saw them..."

The phrasing of "which was the globe, which the cube" presupposes that he is able to visually distinguish the two shapes sufficiently. If he can distinguish the shapes, this implies that he is able to process the visual input already enough to understand that there are two shapes. Given this, I can't understand how a sufficiently intelligent person would be unable to tell which is which through logical deduction.

Besides which, it would be a pretty lame philosophical question if the man couldn't even process the visual input to know that there are any shapes at all.