I believe in one of the NTSB briefings it was said that the crack was at the rear and would not have been possible to see on a visual inspection from the front (I think it was in Q&A and that was the question).
Actually, what he said was that the crack was found to be in the
interior of the blade. This was in Sumwalt's briefing yesterday afternoon (post 250), at about the 20:00 min mark. He rolled up a piece of paper and pointed inside the tube to show that it wasn't something that would be spotted in a simple visual inspection.