Simple answer, it was Charles A. Kiesling Sr. He was my father and he did indeed write the code for the blinking cursor when he worked at Sperry. He passed away yesterday in Minneapolis at the age of 83. I remember him telling me the reason behind the blinking courser and it was simple. It was not because it looked like an "I". He said there was nothing on the screen to let you know where the courser was in the first place. So, he wrote up the code for it so he would know where he was ready to type on the Cathode Ray Tube. It ties in with this patent for the display stuff he put together for the screens back in the day. http://www.google.com/patents/US3497760
And a foot note to this, he was not happy when the first Apple computers came out and they had is _ blinking on the screen. Since he worked for the company, he let Sperry deal with it as he wrote the code for Sperry.